Let me put it this way- how much are you spending on this conversion? Is it worth spending a few hundred quid by getting your own engineer?
If you still feel it is not worth spending the extra then check the engineers qualifications, make sure that the cert is RIAI or CFI or Law Society standard and get a copy of their PI insurance.
BTW you should really have agreed the cert to be used and checked this guys qualifications and pi insurance before you accepted him.
And I would still advise you get your own engineer to doublecheck at this stage.