I thought I would share this repair as some may find it useful. The first photo shows the state of the thermostat on the paso I am currently reinstating. As you can see the steel ring has corroded and allowed fluid to leak. I did pick up a second hand aprillia thermostat and planed to mess around with the pipes to make it fit but when I noticed the steel ring was starting to corrode and I figured soon enough this would also fail. I have a lathe so decided to make a ring to replace the old one. See photo for dimensions if you have a lathe or a friend that does. Note the dimension x is not important - a few mill is good. I then sealed the gap between the two halves of plastic with high temperature auto motive silicone sealant. ( I used the expensive German brand - beginging with W I think?)
I then carefully peaned the edges down with a small hammer ( also filled the void with sealant).
Finally I clamped the ring in the vice going around progressively clamping the whole thing together. I have only had it running for about ten minutes in the garage but so far it's working OK. I will report back later after I have used it in anger for a bit longer. Now back to getting her running as after 5 minutes of clean running she is spluttering! One step forwards two steps back.
Hope this helps and keeps a few more running!
Keith mm in SE London village.

Please let us know if the fix works and I will add it to the FAQs.
It`s a bit sad that helpful posts like this remain unanswered. I appeciate it a lot and would wish there would be more posts like this.

Hi
Thanks for your comments. It's working so far! The paso has not left the bench but I have been messing around trying to get the dellorto/pump/regulator set up fully functional. It's been up to temperature (fan on) and cooled down about a dozen times so far. Of course the real test is on the road. This I will report back later. I'm pretty confident it will hold as the original holding method for the two halves is a crimped ring and I have just repeated this but with additional appropriate sealant. If I had time and the inclination I could make a crimping tool to pull the ring down- but I don't!
Hopefully I will get the bloody Weber to work- failing that I have a pair of very worn dellorto 40mm carbs that might get thrown at her!
Regards
Keith MM