Hi foggy, I had exactly the same problem with my 1.8 Supersport that took me nearly a year to cure. The car was virtually impossible to drive slowly due to a constant misfire but ran fine when your foot was down. I changed virtually everything before I stumbled on the cause. The plugs, distributor cap, leads and rota we’re all replaced with no effect as were the water temperature sensor, the air temperature sensor and lambda sensor. Again with no effect. I reset the throttle position sensor several times and cleaned the idle speed air control valve twice, I also checked the ECU manifold pressure sensor all again with no effect. So with only the ECU and injectors to change, I took the cheapest option and replaced the idle speed air control valve and ureka success and the car has run beautifully ever since. This might not be your problem buts it’s worth a shot. Buy one from eBay for an MGF at about £10 from the people’s republic of China.