It is a major pain in the bum to get the cc changed now
This is the letter I got back when trying to change the engine size on my Sierra.
The DVLA employees are just mindless robots following a strict set of rules.
Show them exactly what they want to see.
A receipt on headed garage paper.
Engine fuel type.
Engine size (don't be too specific as they only like round numbers!)
Date of manufacture of the engine
I finally got my log book back after complying with these rules
Mark