I searched around a bit for you - many talk of it being obsolete and difficult to find. I discovered this though...
November 2008
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POTT 8905371 GAS VALVE 25K491T-47
NETAHEAT 10/16 16/22 MK2 MK2F 392811 358946
Manufacturer's product number : 905371
http://www.partscenter.co.uk have them in stock - but probably over £300.00.
You will have to have an account but any gas fitter should be able to get them for you.
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I think they stopped making this boiler in 1988 and there are other parts in short supply too.
Good luck...
Peccavi, March 2009