Right a little update for you all with some details. RobMC kindley sorted out an original (used) aluminium/steel piston for me, but stock unlike my previously repaired one. When it arrived gave it a good cleanup, fitted the O-ring seal, brass spring seat and washer from my own piston and also assembled everything back together also using my original spring.
After taking everything apart again and putting it back together with the cocking latch the right way up this time the action was smooth and everything worked perfectly with a test shot into a flowerbed.

Comparison without anything fitted. Only 3.68g difference and mine did not have the original pin fitted but a bit of a bodged one.

My Piston: 149.48g (weighed after it broke and possibly a bit underweight)
RobMC's replacement: 153.16g (no repairs and cleaned)

I have been down the range this week and completed 3 rounds of competition targets that had to be handed in this week to avoid a forfeit. After having a month off with no rifle to keep my eye in while this was going on I was a bit wobbly but that will get better again. However my Weihrauch still appears to be performing very well with no issues from the failure. I will be stripping everything on a regular basis though to keep an eye on how the new piston is holding up! Still 14 rounds of targets to complete for the winter.

I will confess that I do use a 3P shooting jacket and a glove to rest the rifle on my left palm, but otherwise just jeans and trainers. No fancy headgear, trousers, blinders, modern sights palm rests butt plates etc etc.
10m standing using NSRA paper targets (5 on a sheet moving around the card)

Would be interesting to see if anyone else it still using a Weihrauch for competition and what scores you are getting. I presume I might be the only one still using a springer in the Middlsex and Surrey county competitions