Well, some progress. Winstanleys has been pretty good after a bit of proddding and has contacted Magura UK. All parties have agreed that I can send back just the sliders for inspection. That's great for me as I can just the sliders from my other, older Thor and so can carry on using the fork. We'll see what happens. I also had a chat with a chap at Zyro (now UK Magura distributor) who was very friendly.
All I can say regarding the cause of the cracks is that I didn't overtigthen the bolts. I can't prove it, but there you go.
The idea that the calliper has taken a whack on the side doesn't stack up either. The calliper is unmarked, straight and is anyway well protected by the fork leg.
I have been using Magura products for 15 years from the early rim brakes through to the current stuff. I've never had a problem with any of it until these cracks appeared.
Thanks again for the responses.
Rich