I just recently had an MT10 timer fail on my Greenstar combi boiler. It looked easy to replace so I sourced and ordered one from an internet supplier. It arrived quickly and I fitted the new timer without problems but it didn't work ! It had a different failure mode to the old one and I checked its operation using a test meter on the connector pins. It definitely wasn't working correctly. So I'll send it back to the supplier and ask for a replacement. Then again, maybe I'll source and fit a better type of timer switch.
Graham.
P.S. All the dire warnings on this forum about gas escaping round the cover are rather 'over the top'. There is no danger of that. You only need a small amount of common sense and replacing the timer switch is straightforward. It's a shame the replacement timer switches have a poor reputation.
Graham Beamson, November 2016