After having a less than satisfactory experience elsewhere I decided to try Northampton Motorsport last October. I was there on time but was kept waiting for half an hour while they had their lunch. I don't think they had any intention of starting when I had booked in. Anyhow everything else about the day was fantastic. I was able to be in the workshop while they did everything. I thought they'd use an exhaust probe but as I have an air/fuel ratio gauge they took the oxygen sensor out and used their one for a more accurate indication than using the exhaust probe. They were very thorough and it was money well spent. The carb was as it should be and they were able to quite quickly dial in the ignition map to bring up the mid range slightly. Very pleased with the standard of work and would recommend, only let down by being kept unnecessarily waiting at the start.
https://www.northamptonmotorsport.com/