Firstly. These are based on my experience.
The V5 Document
DVLA website link
The DVLA state on their site that you will need written evidence of any changes to engine number and cylinder capacity (cc).
Written evidence can be:
* a receipt for the replacement engine
* written evidence from the manufacturer
* an inspection report provided for insurance purposes
* written confirmation on headed paper from a garage if the change in engine size took place before you bought the vehicle
What the DVLA are basically looking for is confirmation of the engine number and size in case the engine has came from a stolen car and had the engine number altered.
The easiest option for me, from those above, was to get written evidence from Vauxhall since I bought the engine 2nd hand.
How did I get the written confirmation? I contacted Vauxhall customer services directly on 0845 0902044
I just called them up, explained that I had changed the engine in the Corsa and that the DVLA were looking for me to get a letter on Vauxhall headed paper confirming the engine number, size in cc and fuel type.
The lady that I spoke to couldn't have been more helpful. I wasn't charged a penny.
Once you have done that send away your V5 with the new engine number and engine size details along with the Vauxhall letter and a covering letter.
The DVLA didn't request any form of check to be done on my car but you never know.
If anyone has any questions please post them here rather than sending a PM. Thanks.
Thanks to Dave Dempsey for the tips too.