I have made detailed reference to this concept before - see September 2011 and December 2011 Bulletins. It is really important because HMRC like to raise assessments for earlier years on the grounds that if a taxpayer has made an error in one year, then it may be presumed that he made similar errors in other years.
HMRC are naturally very keen on this idea but they have been pushing it too far and have been rebuffed time and time again. The authority they rely on for this presumption is Jonas v Bamford (1973) but this case does not give them this authority and the Courts keep telling them so - for example in Syed v HMRC TC 1776 and Chapman v HMRC TC 1593, and now in Aeroassistance Logistics.
However, they are not discouraged. In Aeroassistance Logistics Limited v HMRC TC2628 they again argued the same point and again the Court told them that the argument is wrong.
There is something rather unsatisfactory for HMRC to keep advancing an argument which the Courts keep telling them is wrong. I suppose that they hope one day a Tribunal will find in their favour. But what good will that do? One decision in their favour and a whole load against them. If they feel so strongly about this point, they should organise a change in the law rather than keep arguing that black is white.