Mine has the same problem. I changed everything, plugs, leads etc then a new cat and passed the MOT but still it was still running rough. I then changed the Lambda sensor and since then the engine is running perfect! Get a good Lambda sensor though, I got one made by NGK. The last one was fitted by a supposed VW specialist called Streetwise in Bristol and that failed within a year, together with the new cat they fitted as well! Mine was getting something like 32 mpg but it went back to over 43 mpg after I fitted the Lam sensor. Good Luck