I think you have got a reasonable deal. Changing a pan over as a one-for-one replacement isn't a big job - an hour max, perhaps, but you will get stuffed by minimum job times (because the biggest element unless you have a very local plumber is going to be the travelling time to get to your door).
You amy be able to get the whole job done under the accidental damage clause of your buildings cover, but there is going to be your excess to pay. Compare that to what labour price the plumber wants to fit it.