My 1999 fiesta is over revving, whats the fault?

Hi, I have a 1999 1.25 zetec ford fiesta ghia. I have had the car for 7 years with no problems until 18 months ago the car died and I had the ECU replaced. It had been very reliable so I felt it was worth the £600 to repair it. Since then it has run great and seems much more economical.
2 months ago I had the timing belt replaced (80,000 miles clocked) but since then i have had the over revving problem. It occurs when slowing down when the clutch is dipped, the revs become excessive and is noisy. The noise can be quite embarrassing. The revs settle down once the car has stopped and the clutch has been released. I had a word with the garage that replaced the timing belt, but I was told it was not related and was a common fault with a fiesta this age. Is this true or can there be some conection ?
I have read on a forum that unplugging the battery for 10 mins can solve the problem, is it worth trying this as I can't find my radio code.
I am a community midwife so use the car daily, and do a lot of local driving, and i am gutted as I love my old fiesta. I know nothing about cars and do not want to be ripped off at the garage. I would be very grateful if you could give me any advice ?