I recognised everything you said Outtheway, except that my father's orange engine warning light kept coming on. If the car stalled, he had to sit for 10 minutes or so before it would start again.
Major problems are usually easy to diagnose as they are obvious, but something like this is tricky as it could be many things. It's just a process of elimination. The garage you are using sound very helpful and they are obviously prepared to help you keep the cost down so I would stick with them. If they keep testing various possibilities it won't be long before they find the problem.
Hope you get it sorted soon.