Although I could manage, I decided to do something to move the spring anchor point forward as suggested.
Well, I reckon I've spent about 15 hours making a bracket that does the job! I thought there would be a simple solution but there wasn't.
The new spring anchor point looked like it needed to be exactly where a bolt passes through to hold a complicated plate with the existing anchor point on. I made up a special bolt to fit in the bolt hole. This worked in that the stand would now stay down but when retracted it didn't feel secure. I was concerned that I could go aver a bump and the stand could flip down!
Second attempt involved welding a bent stud to a replica of the original spring plate. The bend would bring the pivot point up as well as forward so that the spring would go over centre both when the stand was up and down. This worked but the spring was now too short so a tab was fashioned to extend the length of the spring.
However, this plate wasn't quite right and so a final version was fabricated this time using 2 bolts with their heads chamfered then welded together and to the replica of the old spring plate. Old arrangement below, my new version above.
Now the stand stays down and stows away securely.
I'm not sure if it would flip up if I rode off with the stand down and it caught on the ground. I'll just have to make sure I remember to raise it. I didn't have any problem on the S3 with a cali stand and no interlock switch.
The only thing I might need to change is to cut the extra length off one of the threaded sections as I can see it will get in the way of removing the sump.