Any question answered...

You did take the grub screw out first, didn't you? Unfortunately, even if you did, the grub screw distorts the threads in the centre bolt, so it fucks the soft female thread in the brake centre-arm as you take it out.

I did but it didn't make any difference.

What's the best cheap solution to this? Buy another brakeset and fit the front on the rear, and try and sell the 2 rears that I have?

I really should have just splashed out a couple of quid more and got the Tektro R359s from Ribble. Might try out Wiggle's returns policy