Thanks for your reply Matt

I asked the shop to check the EGR valve ( i read some posts about that) last time, but the mechanic said the engine is too smooth and powerful for that to be an issue (so he didn't check!). Unfortunately, I don't have the skills, space or tools to do it myself. The O2 sensor was changed a year ago. The mechanic wants to wait a few tanks before he tries that (so the exhaust can clear of soot). He thinks it might be the control unit, but is ruling out cheaper options first.