Very clearly set out what to do on the V5.
Space for engine number and another space for cc.
you just fill it in and send it off.
I tried that last year when I changed the engine on my Sud. Sent in the form with the revised CC and engine number. I have fitted a 1600 engine to Sud 1.5 SC which moves it up to the next tax-band. The DVLA V5 notes said that I only had to prove the engine details if I reduced the engine size so that it changed the taxation class.
Wrong! I got a letter back saying that the V5 notes were out-of-date, the rules had changed but they hadn't bothered to get the V5 notes re-printed to match the new rules.
I needed to prove that the engine was what I claimed, suggestions included a receipt from the seller, an AA/RAC inspection or a garage report.
The engine was out of an old Sud rally-car I had bought about 5 years previously off Ebay so no receipt. Somehow I couldn't see how an AA/RAC inspection would help and how would a garage prove the engine size without stripping the engine?
As an alternative I got some background info off the t'Inter-Web with the help of Google. I explained that the engine was a combo of a 1.7 block and 1.5 crank in order to meet rally engine class size of 1600cc, found a site which explained Boxer engine code lists and code-letter on the top of the block for a 1.7, photographed the engine number and block code on my engine. I also mentioned that the engine had been bought 5 years previously on Ebay and that I was actually volunteering to pay them more road tax, not less
To my relief and some surprise this was accepted.
I do wonder though whether all DVLA employees would be so reasonable!