Got a response from the heat sink seller. I can get the Coolermaster Hyper TX3s for £10.80 each. There will be postage to my address and then postage to you added on top, but I'm not sure how much that will be.

I can't do the drawings for the mounting brackets until I get the heat sinks, so it'd be over a week between me getting the heat sinks and receiving the brackets, unless somebody on the forums with a TX3 and some experience in CAD would be kind enough to draw up some brackets for us?