As suggested Si, there are a few fundamental test that need to be performed but you need the correct kit and quals to be able to get into that part of the fault findingWhat are the couple of things you think it could be and is the pump one of them?
The way you mention that it seems like that happened when the BG guy was there? If the boiler got up to 95deg with him there and he said there was nothing wrong then you need another engineer in.After 10 minutes or so it cut out but that was at temps of 95° or more
So that isn't an overheat point so it won't be the temp that is shutting the boiler down unless there is a faulty sensorit gets into a cycle of heating to 65 then cutting out
If you are seeing a gradient limitation error then it could be a circulation/pump issue but that would normally be associated with an overheat condition. TBH you really need an experienced engineer that knows his diagnostic procedures to go through it and check everything.