Chain Woes - Where to look next...

Looking for some advice, did the Polaris on the weekend and broke my chain on the first day, not a big deal, did a quick repair to get back to base and although it broke again just before I got back it wasn't hugely suprising as it was only a quick repair.
Overnight I got a new chain professionally fitted and gears adjused etc, so imagine my dismay when on Day 2 the exact same thing happens with the new chain and breaks in exactly the same way (Insiide link bent towards the bike)
. Only conclusion is that there is a problem in the drivetrain so I have inspected the chain rings, sprockets, jockey wheels and no obvious bends etc and they all run true.
Any advice on what to replace in which order or where to look next?
Tim
Overnight I got a new chain professionally fitted and gears adjused etc, so imagine my dismay when on Day 2 the exact same thing happens with the new chain and breaks in exactly the same way (Insiide link bent towards the bike)

Any advice on what to replace in which order or where to look next?
Tim
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Parktools :?:SheldonBrown
The bike is a Cannondale Jekyll. I can't remember what chain I had on before, I think it was a Shimano, now it has an SRAM 971.
Bizarrely both times I had just come to a road section and was only just getting started, no extreme presure, previously I had been climbing an decending around the Seven Stanes Glentress region and all was fine. Also was not changing into gears that I hadn't been previously using.
Thanks,
Tim