A dedicated band of volunteers is prepared to put itself through everything from a suicide attempt to a bus crash to help first-aiders and hospitals hone their skills.

Caroline Thomas has had a heart attack in accident and emergency, an epileptic fit in a GP surgery and been cut out of mangled car wrecks and treated by paramedics more times than she cares to remember. Severe nose bleeds and dizzy spells happen all the time, and she has suffered the whole gamut of serious injuries from third-degree burns to an open fracture of the tibia. But the reason her diary is such a catalogue of disaster is not that she is accident prone but her connection with a quirky organisation whose members combine an expertise in first aid with a fascination for drama and make-up.

The Casualties Union is a band of some 900 volunteers who simulate accidents to allow people to practise first aid on them. The organisation is a registered charity, and those who make use of its services are charged only for travel expenses and the cost of materials. Clients can be anyone from St John Ambulance cadets to hospital staff testing major incident plans. Most members have a background in first aid, usually with either St John Ambulance or the Red Cross, and range in age from eight to 82.

'We're quite a mixed bunch,' says Caroline, a former bank worker who took early retirement and now runs the Casualties Union London region. 'I joined up partly because it appealed to my artistic side, but also because I thought it would be doing something useful.'

Doctors and nurses are brought in to train members so they can accurately portray the symptoms of any condition. They must also pass a practical exam to ensure both their acting and make-up techniques are up to scratch. 'You have to act out a scenario,' explains Caroline. 'So if you're in a church hall, you might use the kitchen area to simulate an accident with a knife, or perhaps a burn.

'You do the make-up yourself and then play the part to make it as convincing as possible. If you're unconscious you must be totally limp and not move.'

Members go to great lengths to achieve realism, applying gruesome wounds and even burning their clothes to get the right effect.

One of Caroline's favourite scenarios is an open fracture. 'I find Marks & Spencer's Chicago spare ribs are particularly good for this. You boil up the bones, dry them and smash the ends about a bit with a hammer to get a nice jagged edge.'

The next step is to cook up some fake tissue in a saucepan using a mixture of oil, flour, salt, cream of tartar, food colouring and water. 'As I have freckles, wholegrain flour is the best for me,' she says. 'It's just like making a sauce - when it comes away from the pan you know it's done. If you get the salt wrong it'll grow a culture, so there's quite a skill to it.'

The end result resembles Play-Doh, and, using diluted glue to make it stick, the tissue is applied to the skin using a pallet knife before the smashed bone can be added. The key, according to Caroline, is to blend in the edges so there is no visible join.

'My other specialty is ruptured varicose veins, which is a popular test for first-aiders to see if they know how to staunch the bleeding,' she says. 'We rig ourselves up with intravenous drips filled with fake blood hidden under our clothes. You give them a little squeeze and the blood pumps out. With venous blood it's important to get that darker colour, so I make sure I add some black ink.'

Caroline admits to getting engrossed in the whole process. 'I was dividing up an orange once and I cut myself quite badly, but I was so interested in the colour of my blood I had to go off and compare it with fake blood. After a while it occurred to me I needed to treat my cut before I bled everywhere.'

But sometimes the realism is a bit too much for the would-be first-aiders. 'When a less experienced exam candidate was dealing with my ruptured varicose veins he said to me: 'I don't think I can take much more of this'. On another occasion, I had to sit out under a tree and pretend I'd been bitten by a snake. When the candidate came up and asked me what was the matter I gasped: 'snake bite'. She looked absolutely horrified and hurried off. It turned out snakes were a particular phobia for her and it took quite some time to reassure her there was no snake, not even an artificial one.'

Casualty Union members are occasionally asked to engage in some hair- raising stunts, so what is known as the 'no duff code' is important for their protection. Caroline explains: 'I could be lying under a load of rubble - in a metal tube for protection - and someone might be fitting me with a surgical collar but compress my windpipe while they're doing it. So I'll say 'no duff, you're hurting me' and the exercise will be stopped until it's sorted out. Then you go back into role again.'

Caroline adds that the one thing she lacks is the experience of having been in a real accident.

'Sometimes it's hard to imagine how you would feel if an overturned bus was on top of you and everyone around you seemed dead. You have to act in the way you think you would in reality and I tend to be fairly calm. But some of our more highly strung members are prone to go into hysterics.'

Christopher Gundry, health emergency planning adviser for south London and Surrey, says he finds the service provided by the CU invaluable.

'If you can send in a large number of casualties within a relatively short space of time, as if they had come from a major incident, it gives hospitals a far better feel for what it's like to deal with the real thing. Staff have to handle people behaving like real patients behave - exhibiting signs of confusion or anxiety, with some being co-operative and some not. Using CU you get 10 people and all the complexities of coping with them. And because the 'victims' have a knowledge of first aid they know what treatment they should be getting so they can make valid comments on what was done to them.

'The realism and the pressure people have to work under is useful from an administrative point of view. And from a clinical standpoint it allows staff the chance to practise their triage skills. Many hospitals use the exercise as a teaching opportunity and take the casualty through to theatre and say to the doctors: 'Here's a serious head injury, but you have 10 more like it outside. What are you going to do?' The people's ability to simulate injuries has to be seen to be believed.'

Members are very serious about making the experience as realistic as possible for those who are meant to be learning. 'If I'm meant to be having an epileptic fit and someone sticks their fingers in my mouth, I'll bite them - not hard, but enough to let them know that it's not a good idea.'

On the BBC's Casualty series some years ago, a storyline was included about the CU in which, during an accident reconstruction, the CU members didn't realise that one of their number was really having a heart attack.

'We were very unhappy with the way we were portrayed,' says Caroline. 'Our exercises are carefully supervised and that just wouldn't be allowed to happen. There also seems to be this misapprehension that we're all failed actors, but it's just not like that. We're ordinary people from all different walks of life who get together for a fascinating and satisfying pastime in which we provide a service that one day may just save a life.'