I've had the same issue with my S3. After a lot of research I've found the manufacturer in the US and am just about to order one. It's $177 including DHL but not clear about whether any further import duties will need to be paid. They have 'em in stock and can get them over in a few days.
The part is Electronic Heater Control Valve 354-69494 from Thermotion. I've been emailing a helpful chap called Chris Swanson (sales@thermotion.com). The price of the part is a little under $100. Last price quoted by Morgan was a lot more! But I don't know what it is right now. Shame about the dollar rate!!!
I bet I could order 2 and save a good deal on the delivery and then send one onto you. Happily split the total bill with you plus whatever delivery to get the part to you in UK. Should sabe us some Wonga.
Brands Hatch Mogs removed the old part for me and I have it at home. Looks like a little grand piano and doesn't take long to fit...
