GC10

Hi, I hope someone can advise? We have an upstairs radiator valve that is leaking and has corroded the half inch pipe running up to it. The central heating system has 2" pipes circulating the house, so not sure it can be isolated. We've had a quote for £465 (inc VAT, labour and parts) as it was said that the whole system would need to be drained and refilled. Can anybody advise whether this sounds right? Many thanks GC