My question is, do I need to worry about any possible leaks occurring
after the pump is switched on, or is it the case that if there are any
leaks (i.e. incorrectly plumbed), that they will show up whether the
pump is on or off. If its possible, I obviously need to have it
working while he is there, so he can fix any leaks.

I'd be worried about not testing a key component like this. However, can't
you simpy wire it up temporarily? Just stick a 13A plug on the end of the
shower pump wire and plug it into an extension lead.

Cheers, Rob.