Been out tonight fiddling with the car, decided to start it to see how it was running. Started and stopped it 4 or 5 times without a problem. Then tried to start again, car started but the starter motor was still running.Turned the key to off and pulled the key out but starter carried on running. Grabbed a spanner and started undoing the batter lead. While doing this the engine was going slower and slower until it eventually stopped, before I could get the terminal off. Checked the switches and connections with a multimeter and all seemed find so I reconnected the battery and tried the key again. All started and stopped fine.

I'm guessing the solenoid had stuck but the question is why and what can I do to prevent it happening again. Forgot to mention engine is an 1800 pinto.

Assuming the switch is OK it sounds like the pinion or fork is sticking, remove the starter and remove the solenoid from it clean the mechanism with WD40 making sure it slides freely ---don't oil as all the clutch crap will stick to it and jam it.

Might be worth fitting a battery cut off switch as well. just in case

Your dead right rustynuts, I think a cut off switch is essential. I always turn it off when I leave the car. Also have it where it can be reached from the driving seat.