As Danny said its more than likely going to be the rocker cover which is leaking and mis-diagnosed and needing a re-seal. Whilst you're having it done (properly at least) you might as well treat it to a timing belt, aux. belt and (service history and condition depending) water pump change.
Obviously we've not seen the car so cannot say for sure but if the above is correct i'd charge £410 - £467: That breaks down to £380 to change both belt kits, and another £30 to remove, chemically clean your covers then refit and reseal them. If you wanted a water pump adding it would be an extra £57 with type-D coolant following a flush.
Mick