Hi
I’m really sorry to read of these problems. I've worked as a technical specialist for the AA for many years and have dealt with lots of complaints from AA members who have been sold poor quality ‘reconditioned’ units. The cases have often been very difficult to resolve.
We always recommend buying from a member of the Federation of Engine Remanufacturers http://www.fer.co.uk/ .
If you plan to have an engine fitted by a garage it's much safer if you ask that garage to deal direct with the engine supplier and you pay the garage for the whole job. This means you’ll only have a contract with the garage and if there is a problem with the work or the engine it'll be much easier to deal with one company. The garage and the engine supplier can sort out any difficulties between themselves. It’s very difficult to determine who is at fault if you weren’t involved in the repair. If you supply the engine for a garage to fit, and it's then found to be faulty you'll have to pay the garage to remove it again. Theoretically you should be able to get the money back from the engine supplier, but from experience, it's not that simple.