High tickover on Fiesta 1 4 Zetec.

[scs1]

Can anyone help?. My run around is a Fiesta 1.4 Zetec and my usual mobile mechanic recently replaced a water pump and a cam belt Ever since, when I stop ,the tickover is up around 1600 for about 5 seconds until it drops back to normal. The longer the journey the longer it takes to return to normal. My guy thought it was the speedo drive sensor ,but replacing it it has made no difference. He has hooked the car up to his diagnostic machine but nothing obvious stands out. Any ideas would be appreciated.