he should cover this out of common courtesy, have you made all payments to him if you have not paid him in full you could discuss with him the possibility of keepi ng some of the final payment

based purely on the details you have provided, i would suggest that at face value the builders should cover the cost. it would be reasonable to expect the builder to note that the alarm was in full working order, and so efforts should have have been made to protect it or remover and refit it, regardless of the fact that this wasnt specified in the contract.

it is normal for a schedule of dilapidations to be produced showing all existing damaged items prior to work starting, and the builders should do this as a matter of course even if not requested by the client. inspection of this would then show that the alarm was not on it.

it is also normal to include in any contract for the removal and/or protection of existing services, but even if in this instance the alarm wasnt included in this the item should still have been protected for retention or reinstatement

Its always difficult to give a definitive answer as there are generally two sides to a story. One would say the builder should have clarified if you wanted to retain the alarm if it was in good working order.