Something I've done over here in the states (and I see you guys have them over there in the UK as well) is hire a pre-purchase or used car inspector. When I looked them up the other week over there they run around £100-150.
You contact them with the type of car you are looking at, where its at, if you have any specific concerns about that car. Then they go out to the car lot or to the individuals home and conduct their inspection (typically unannounced if its a car lot just as if they were any other person going out car shopping. The reason is they don't want the shop "prepping" the car and warming it up).
They hook their computer up to the car and see if any codes pop up, they do a REALLY thorough inspection, take it for a drive and then they give you their full report about it. These have been so valuable to me when buying vehicles.
Over here in the states I've used them about 5-6 times for different vehicles I've been interested in purchasing or have purchased. A couple times they have steered me clear of bad cars. Even when the car checks out ok I dont feel like I've wasted my money. It gave me a very sense of comfort.
These are just a couple that I found over there. I'm sure there are more.
http://www.nationalvehicleinspections.com/prices https://www.mycarinspections.co.uk/book-a-service.php?stage=2