Method one is to look at some estimates of total wealth, impute a return and then say tax should be paid on that imputed return.
However, some (much? Most? A teensie bit?) of that return will come in the form of unrealised capital gains. Indeed, amongst billionaires you might expect a lot of it to come from such.
We do not tax wealth, nor do we tax unrealised capital gains. Thus this method of estimation of losses fails.
Method two is to refer to his own report The Missing Billions. Which as I\’ve noted before is logically flawed. You cannot estimate tax losses by looking at the difference between headline tax rates and actual tax rates. For Parliament deliberately and specifically includes schemes which reduce tax rates from headline ones.
And somebody used this "research" as the basis for a TV programme.