https://www.glow-worm.co.uk/trade/service/fault-finding-codes/subsites/betacom-a-range-6-232598.html
https://www.diynot.com/diy/threads/f25-error-on-boiler.319077/
https://www.justanswer.com/uk-plumbing/7frp8-error-code-f25-circulation-problem-glowworm-ultracom.html
Bags more advice out there for those prepared to search for themselves. I used:
"Glow worm ultracom 38 cxi f25"
aslkjklafs
February 2018