Just how much does a GP earn? The NHS Information Centre estimates average GP income in 2008-09 (the latest figures available) as £105,300. But any average conceals variations.

GPs with an estimated income of more than £200,000 numbered 950 (2.9 per cent). Meanwhile an average salaried GP earns £57,300.

But the real answer? It is akin to how long it takes to tune a 12-string guitar: nobody actually knows. Most GPs are contractors to the NHS, meaning they are partners in independent businesses.

They draw salaries, but their total earnings are a complex mix of dividends, expenses and business growth. Essentially it is between them and the tax office.

Besides, the general medical services contract is complex and at times opaque. At its core is the GMS “global sum”, for routine primary care to a registered list of patients, which brings in £63.21 per (weighted) patient. That is around £250,000 for a 4,000 practice list. Then come payments for GP seniority, enhanced services, the quality and outcomes framework…

Primary care trusts have found it hard to control this cost - unsurprisingly, some would say, given their governance structure. But GP pay is under increased public scrutiny. Last week’s HSJ report (news, page 4, 3 March), showing just how GP-controlled commissioning funds are being spent on refurbishment, medical kit etc - allowing practices to boost their profits - was immediately reported in the national media.

Last week also saw a more direct threat to GP income. Exactly how robust are those list sizes? NHS Brent has annoyed local GPs by taking up a private sector company’s “added value service” of “national list cleansing”.

“If you do not complete and return the form below”, their letter to patients states, “your GP will have six months to confirm your details. If they do not do this, you will be removed from your GP’s patient list.” It requires evidence: a letter, an appointment, a prescription or a phone call.

Within six months: in Brent, with some 120,000 names under scrutiny, that is a lot of practice £63.21s at risk, and a lot of extra work for practice staff.

The company? NHS Shared Business Services.