Thanks elekie.

No idea what the fuel pressure is. When I said a misfire, it's more of a 5-10 second period of lumpy running which then clears.

Garage said they changed the filter. She's going to take the car back for further investigation. Whilst the garage aren't Mini specialists, the particular mechanic was recommended as being knowledgeable on them.

Elekie. You mentioned fuel pressure upthread. What range is acceptable?

An update.

The car went back to the garage. It appears it was the high pressure pump which was rebuilt first time round.

After further investigation they said the low pressure fuel relay was running high resistance causing it to not work properly. Another problem they found was the low pressure fuel pump was not working efficiently and isn't feeding the high pressure fuel pump the pressure needed or desired by the ECU.

They offered two avenues to fix, both involved replacing the low pressure fuel pump but the relay repair is where they differed. Either replace the relay within the board (more costly) or introduce an external relay to bypass the internal relay (cheaper).

My friend (who knows little about cars) went for the cheaper option on the understanding it would fix the problem. The garage were "fairly confident".

Nearly two weeks on and the garage report the car still has issues. They've recommended an auto electrician takes a look. My friend doesn't know what to do next.

Wise words elekie, but finding a garage that knows what it's doing is not easy.

This Mini is 13 years old and hardly the latest tech. Surely most competent mechanics will have worked on them by now.