Contingent simply means conditional or dependent.
The first point, I think, is that the OP described the loan note consideration as conditional, but did not say on what
it was conditional. About 10 years ago, I had a 6-month argument with the Revenue about s48 and Marren v Ingles on a closely related matter - retirement relief - and I won the argument. The distinction is this:
If I sell my shares to you on terms that you will pay me £1m in 5 years' time, that consideration is ascertainable. In other words, I know exactly what I will get for the shares. The risk that you will renege on the debt is deliberately excluded (by s48) from consideration.
If I sell you my shares on terms that you will pay me £100,000 plus x% of the profits over a certain period, that consideration is not ascertainable, because it cannot be calculated with certainty
upfront. I think we're agreed on that much, at least. However, following Marren v Ingles, a best estimate of those profits has to be made and included in the CGT calculation. This consideration is not ascertainable at the time of the original disposal, which is what I meant.
If the proceeds I receive are contingent on anything, that implies that there is a risk that the figure will be different from that originally estimated.
Hence consideration cannot be both contingent and ascertainable.
Ascertainable is defined as capable of being calculated, either by taking a known figure or drawing up a simple account to establish such figure.
Defined where? In any event, if there is uncertainty as to the amount of the consideration, then it cannot be ascertained (which implies certainty), only estimated.