I'm on Talktalk and I'd like to consider a move to BT. I've got TT Fibre, so want to move to Infinity.

Sometimes the BT checker says I can have Infinity, sometimes it says I can't have Infinity. Just what is going on? Chafford is Fibre enabled, and I have had a fibre service for 2 years - so why can't BT offer it?

Sounds like capacity issues where the cabinet gets full,if people have cancelled and there are spare slots it will show as available then not available when these are taken.
Sometimes Openreach wll increase capacity in a cabinet and i believe an "order and wait "process is soon to be available.

If you have TT fibre then a move to BT Infinity wouldn't change your physical connection in the FTTC cabinet so capacity problems shouldn't be an issue, you just need to ensure that the cease of TT and the provision of BT are linked together, I presume you would have to also ditch your TT telephony and take BT for your phone provider, this would probably require a visit to your local exchange to move you from TT to BT , but this would be for the telephony only

This is spot on. Ignore the checker and just order from BT. They will pull the service from TalkTalk and it should use the same port on the fibre cabinet.

thank you