I wonder if this is mileage related because, so far, i've not got (or seen) and cracks but i'm always looking. Normally the cracks go around the weld or stem out from the rivet on the frame number plate towards the weld. This is different from all the others i've seen but you can see it's hairlining to the other side.
Hope you get it sorted Frank. I've got some other pics of the same problem. Do you want me to post them up?
Or is it because you've been doing too many wheelies with heavy landings?