Sounds like you're being fobbed off. Ask a friendly taxi driver to recommend a local diesel specialist to ensure the problem is sorted. Personally, I would never buy a second hand common rail diesel that's out of its guarantee with no idea how it's been driven from new. The engine sensors tend to get clogged up with carbon deposits especially when driven predominantly on stop-start trips and you can't tell if it's ever been misfuelled by previous owners!
MINI has revised the Countryman with slight tweaks to the exterior styling and improved engines for better fuel economy. The revised car is priced from £16,990 and will be available to order from June.