@auld duffer Thank you for the summary.
Re:

cappers said:

Further thoughts, although most of the 90 is original and unmodified, the engine and gearbox aren't - 3.5V8Efi ex1991Disco and a ZF4HP22 autobox ex1989 RRC. etc........

Click to expand...

When I swapped the 2.5P & LT77 for a 3.5V8 & ZF4HP22 I informed the DVLA and had the V5 amended to reflect the engine change; the gearbox isn't mentioned. I informed them that 3.5 V8s were an option for 90s of this era and it went through without a quibble.

Since the V5 registers the 90 as a V8 then the points system won't apply so theoretically I should be able to request a number plate related to its actual age i.e. build date rather than date of first UK registration, without worrying about changes.
However searching the DVLA website I can't find anything about reregistering a vehicle with an age related plate, only for rebuilds etc.
Any suggestions appreciated.

Further searching and form v765 looks the way for mine.

Looks like I started something here! Thanks for all the info guys.