When we did ours, a cossie engine into a mk1 xr4x4, i informed the insurance company of the new engine and had to get an mot garage to write a letter on their headed paper to confirm what engine had been fitted. When I sent the logbook back to notify dvla of the change they sent me a letter giving various options to me of providing the details they wanted.
To be honest it wasn't much hassle to sort and only cost £10 for the garage to inspect it and write the letter.