Running like crap.
 

Since my fiance lost her company car, she has tried using my Sovereign a couple of times but complained it wouldn't run properly. I took it out on a fine warm sunny day and had no problems, but fine dry sunny days are a rarity where we live, even if the rest of the country is basking in it. Sure enough the next time I took the car out it was damp, had been raining but had stopped an hour or so earlier. Ran like a Trabant, just not as smoky. Anyone any idea what would cause this

Also, check the plug connectors for the engine ECU for moisture and corrosion. This is a common failure. The ECU is behind the side panel by the accelerator pedal on RHD cars.