Change from Gravity feed to combi boiler?

The existing c/heating is from a Potterton conventional boiler gravity fed hot water from storage tank in airing cupboard and of course the central heating, pumped with two diverter valves in the circuit.
British gas who service the boiler annually say spares are getting scarce and the boiler needs replacing.

Now to change from a conventional boiler to a combi boiler what is involved and what could I expect in cost?
Boiler open wall ie: no cupboards and on outside wall so direct flue feed. all pipework and elecs easy accessable .

I don't want to call in British gas who have offered to price up the job after last years nearly £4,500 less 'carrot' discounts bringing it down by £1,000. and that's a straight change of conventional boiler to conventional boiler a Worcester 'green' boiler.