I think you need to get another spark to inspect the property - then use his finding to request a refund OR face action in the small claims court
As for the lack of certificate - this is something he should issue, outlining the issues he has suggested.
This is all very basic stuff for...
Doing a basic Zs is always a good idea as part of the change. Not only does it check the property is earthed it also helps confirm polarity.
Best make sure your business insurance covers you for all the tasks you think you will be doing
I word listen to the customer and then draft up an outline spec, room by room adding items in red that they may not have thought about, such as outdoor electrics, loft electrics etc etc with a rough price
Then get them to comment