if it is wear in your gear selection then a new set of clutch discs are less than £300.00, with the seals and bearings I would be surprised if the bill was any more than £600.00 in parts.
If they are right and its worn the gear as well then the £1500 might be about right however I would be surprised if that was the case, indeed if it was normal for the gears to wear I would expect the service exchange price to be double the parts costs, i.e. £3000.00.
I would give the guys at RK Marine at Swanwick a ring and ask them what they think, at least then you will have an alternative price.
Hope you get it sorted painlessly,