Hi Emily
In my humble opinion, a training certificate is limited in its use as far as evidence for an NVQ goes.
Some just confirm that you attended a course, nothing more. Just because you have attended a course on, say, hygiene, that does not mean that you carry out your job in a hygienic manner.
If there was an exam involved that could count towards the knowledge and understanding but which particular questions?
I know for the IOSH (health and safety) a safety audit is carried out by the candidate and a copy of that is good sound evidence.
I favour getting practical evidence of performance during the NVQ rather than rely on certificates.
As I say, my humble opinion.

Dave