Fit a new gasket and try it as for £800.00 seems very steep id ring and ask how much the part is and either fit yourself then get re gassed or take it to another garage.

Out of warranty?

No help for your immediate problem but have you been using the aircon for ten minutes every week? Running air con (even in the winter) keeps the system lubricated and stops seals drying out.

My 2p would be to try a aircon specialist yourself. That's probably what the dealer does anyway, might be cheaper to go direct.