Consider having a buffer store near the panels, into which you circulate the 30-150C water via a solar coil (maintaining a small volume of solar fluid).

From this buffer, you pump 40-90C water via plastic pipe - using a motorised mixing valve (of inflow + store) to limit the return temperature to 90C, and a cheap solar controller to only pump if >40C and hotter than receiving store and receiving store <90C.

I think they're all compression fittings tony. Generally solder isn't used in ST.

Personally I'm not too fond of the compression fitting for SS corrugated solar pipe. They rely on silly little fibre washers for a fit. Not a good design IMHO.

Another idea, is to use the working SS pipe for the hot return, and add a plastic pipe for the <90C outflow.

If you wanted to be really funky you could use just the single working SS pipe, and via large expansion vessel or header tank, and valves in the right places, push cool water and receive hot water. You could probably co-opt a drain-down control system to do this.

These are all very helpful suggestions, particularly the idea of using plastic pipe for the return flow: obviously that would be much less than 100C. Threading a smaller diameter plastic pipe through might well be a good idea, when I find out which pipe is leaking (I hope to do so shortly). But the s/s pipe has an inside diameter of a fraction under 17mm, so the largest pipe that could go through would be 16mm: and the inside diameter would be reduced to 14mm: if my maths is right, a 45% reduction in cross-section.

It's quite true the special compression fittings for the s/s pipe are fiddly: you have to use a circlip and the nut supplied to make a flange on the pipe where you've cut it, and then the fibre washer. Unfortunately I don't know where the joins are, and they may be in different places for the flow and return pipes, so a limited dig isn't possible. I tried listening for the leak, the way the old Water Board experts used to do it, but totally failed.

I also like the idea of a buffer store - I do have room for this - and I wonder if it could be made really simple, with the store itself vented to avoid the highest temperatures in the store (nothing over 100C - it would boil). I could then run PEX pipes back from the buffer store to the house: this seem to be OK near 100C (for example, Upanor claim theirs is safe at 95C, with occasional peaks up to 114C). I will keep thinking about this.

Thanks again.

It'd be interesting to know what the working temperatures and flow rates were before this fault appeared and therefore what the losses in the pipes were under different solar conditions.

Your pipe is most likely DN16 which is 16mm ID. No i dont think you would be able to push another pipe inside - at least not without an aeroplane profile nose to deal with all those convouloution ribs.
The joints are fiddly yes, but no big deal really - just make sure you get the apx 2.5mm thick circlips and not the cheap 1.5 thick. Check you have crushed a nice sealing flange and use 2 thick washers - good to huge temps/pressures.
I myself have installed a similar system - ie 120 tubes 50mtrs away from building (installed at 70' for steady daily average gain over extended annual period) and yes intermediate open vented buffer was the solution - and pex-al-pex pipes (availabl;e up to 500 mtrs!) No antifreeze needed underground - works great.
Pex ratings are for suits in brussels and in reality can take higher temps, especially with zero pressure.
The technical limits/ failure is the delamination of layers or plastic softening and this is first an issue at the ends, at the compression joints - especially if brass/ metalic.
Dave Solar Dave - tech at TMS

But if only mentioned once then probably very useful information.