Thanks but I just bought a long one on ebay for £7.36 (including P&P).
Gave up with the header tank and called in the pros... British gas quote Just a boiler: £4500, NPower complete full central heating system boiler rads (plus extra ones in other rooms) pipes fitting etc. £4900. seems quite reasonable fitting in two weeks....
fitted a double check valve on the hot water shower and problem solved, thanks for your help, unfortunately the shower supplier did not put this in the instructions?
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