XT M775
Generally deteriorating performance, excess lever travel and lots of noise then total failure and lever to bar without any braking.
Pads have lots of life left, but filthy, lots of oil similar on rotor – cleaned it all up.
Bled brake, very low oil level.
Lever is now acting well but braking performance still poor, screeching, juddering etc. Checked pads and rotor again and filthy with oil again.
Conclusion – calliper seals gone and leaking fluid onto pads – buy new calliper (or brake). Have I missed anything?

Sounds right.
Just buy new calipers.

Your diagnosis seems sound. As someone who has a set of those and two sets of 975 calipers in use can I ask what mineral oil you’ve been using?

For the bleed I used the stuff supplied by Epic Bleed with their bleed kit.
I’ll look at the cost difference, might be almost as cheap to get the whole system as just the calliper – the bar end is 5 years old anyway

Full set at Merlin £129

https://www.merlincycles.com/shimano-xt-m8000-front-and-rear-disc-brake-set-85820.html