For Information. I spoke to an exhaust centre today who suggested that I could have two problems. Firstly, a fuel problem which was causing the high CO level, and secondly, a leak after the CAT which could allow air to be sucked in by the exhaust analyser. The problem that I have with this explanation is that I would have expected the additional air being sucked in to have weakened the CO levels. Any advice for a frustrated DIYer.