Mike, there are a number of freds on here regarding rebuilds and the dvla regulations. Basically, as far as I understand it, you can rebuild a vehicle, but your right to mainatin the original identity... and therefore the original reg number, depend on the number of original parts and what they are.
When doing the rebuild you will earn points for each part used that is original... or built to original specifications. Hence, if you change a 2 1/4 petrol series engine and put a 200tdi into it you would not get the points available for using the same engine.
Changing the chassis results, as far as I recall, in the biggest loss of points.
If you fail to garner the requisite number of points during the rebuild then you would need to have an engineer's report and apply for a new registration number.
Keeping the receipts for the rebuild does not exempt you from these rules, it simply makes it easier for you to work out whether you have fallen foul of DVLA regs.
If I am inaccurate in this then I am sure someone will correct me pretty quick;y.