Engine Upgrade Specialists

I bought a second hand Caterham Superlight SV R300 (2003 model) last year with a R400 engine upgrade from a dealer in Harrogate. Unfortunately there was no paperwork regarding who carried out the engine upgrade. The car was originally on sale from the Caterham main dealer south. Caterham have not been able to confirm the engine upgrade was not carried by them. My question is who next are the most likely candidates for carrying out the upgrade? I would like to contact these candidates and then supply them with my cars registration and chassis number to determine whether they hold any paperwork on my engine upgrade.

Thanks in anticipation