It's the old 'adequate training' bit of the HASAWA that you need to be aware of, so yes there should be some form of training record for engineers operating the trucks, not necessarily a full operators course but certainly some form of training.

I have run these shortened courses in the past for engineers, and endorsed the certificate with something along the lines of "for the purposes of maintenance, testing and inspection"

If one of th engineers is involved in any type of incident you may need to satisfy the HSE that you have complied with S2 of HASAWA as above, and also your insurers will be asking the same question.

Hope this helps.

(FLT instructor since 1996)