Have you stolen my picture Craig?! That could be a carbon copy failure couldn't it! Glad you got away with it same as I did; mine had managed about 2500 miles.
I realise I never fed back to the forum the conclusion of my wishbone tale. I got a new pair from Marlin, and I spotted that, as Rich points out they seem to have returned to an earlier design where the diagonal is welded right to the tip - here's a pic:
I was reassured by this as it clearly removes the high bending load area that is where our wishbones have failed - where the weld of the diagonal is coincident with the end of the threaded insert. The tube gauge hasn't increased though - I checked by weighing and old and new bone. You'll see that Marlin were nice enough not to weld toe control arm tabs onto the arms (for me to cut off again) since they know I'm controlling the toe separately.
With that I swapped them both and haven't had a problem since, touch wood. The car has never been better since after fitting the new 'bones I've finally been able to drive it with the suspension tweaks. Did 600 miles last weekend without incident - great fun.
I know John thinks I should reinforce them and that may be a wise idea but the design tweak alone I think should have made a big difference. I'm going to be keeping a good eye on them for any cracks too...
Richard