What a great forum this is. Many thanks to PeterG for sorting me out with a pair of original steel mudguards. I had offers from others as well, thanks.
I have modified the stay slightly to (hopefully) prevent a repeat breakage. In the usual Morgan style this has involved beefing the stay up with a sleeve over the original tube, this may just move the next break on to a different position??? I used some 22mm OD stainless steel tube with a 1.5mm wall thickness to get the ID the same as the original stay tube OD of 19mm. It was a tight fit and I secured it with PU18 adhesive, I didn't want to cause another stress point by welding it. Trial fit completed and now off to the painters for some stone chip inside and body colour on the top. I need to get four 30mm long stainless Allen head bolts for the stay joint as well as the originals are too short now.
I will add to this post if things don't work as they should.