How can I stop my Worcester Combi 350 leaking?

My Worcester Bosch 350 wall mounted combi boiler is leaking at the C/H isolating valve (flow) right underneath the boiler casing where the boiler pipework connects to the heating system pipework.This leak occurs when the boiler has cooled down after use.I have to keep topping up the system pressure regularly.Can this be repaired and if so how and will it be very expensive?My boiler is about 12 years old which like myself is getting on a bit now. Thanks for any answers