The EU published new VAT gap data last week for all 28 member states.
The UK summary is here (click the image for a bigger version):
The EU think that in 2012 the UK VAT gap was 10.4% of total potential VAT payable. At 2012 exchange rates the value was £13,428 million (£13.4 billion). The EU says this is very close to the UK estimate.
There's just one problem with that claim: the latest UK estimate of the VAT gap is for 2012-13 (so near enough the same period) and is for a gap of 10.9% at a cost of £12.4 billion. I don't think £13.4 billion is close to £12.4 billion: it is £1 billion.
One of these estimates is better than the other. I know which I am inclined to believe because it was not prepared by the tax authority publishing it, and that's precisely why I say HMRC should not be responsible for monitoring the tax gap.
Note: Updated at 14.30 on 27 October because an incorrect exchange rate was used in the first version