Changed air filter and plugs[number 6 was a bit of a challenge]dismantled exhaust system[it only went on last year and car has hardly been anywhere since then so there were no rust issues]and re-installed it with exhaust paste round the joints-in case there were any small leaks not noticed on M.o.T.] so I think it's now ready for re-test and see how it fares this time on emissions.