Ok just had a chat with the our engineer who works for the DVLA doing this stuff.
It's simple really, you always have to have one original number matching between the Body-Vin plate-Registration documents that is clear and readable. You are only issued with a Q plate if all three numbers (Engine,Body,Chassis) don't match the Registration documents or you don't have a registration document (car built from parts)
It is also true that the same unit number can be used as the Chassis and Body number as long as you can see them clearly displayed on each item, it would only be the number in the Chassis/Frame box of the documents that changes when the chassis is replaced, the third 'Body' number would stay the same meaning you can get away with an Engine swap too if required.
So this is the scenario, at some point in history some one has changed the engine (Disclosed it to the DVLA, Engineer report etc) so that removes one of the three, then you change the chassis which would be the second of the three but the problem arises when the body has been painted etc and the Engineer can't see/read the number on it.... In that case he can't check the third number making it questionable and it would require a Q plate.
Also if you tell the DVLA that a car is a monocoque you might only have two numbers on your cars records, so if you have a non original chassis and you need to fit a replacement engine for what ever reason you might have a problem with the engineer, but if you can find the number molded into the grp you might get away with it.... that said many Elans have been painted and the number is no longer seen.
One thing that is arguable is the screw on plate, some engineers might not look any further that this and if thats the case luck is with you, but allot will see the Engine number, see the plate is not permanently fixed and assume it's a convenience plate for MOT testers then start looking for the origin. All depends on how meticulous the engineer is and what brings you to his door.
It appears there are penalties for non disclosure but i didn't go into that.
The funny thing is the same guy said why don't you just grind the new number off the chassis and stamp in your old one.... Job done