Mk7.5 Fiesta Immobilised

[steve-V8s]

Car started and drove normally, stopped for 10 mins came to restart, battery dead, must have lost a cell as down to 10v. Removed battery and fitted new one, car now will lock and unlock with fob, radio works but rest appears to be immobilised. Message says Immobiliser fault and shows red I symbol.

How do I get going again ?

[twing]

Sounds like the key might be the fault, Ford Customs suffer with the same, everything else ok but won't start.
If this is the issue then a mobile key guy MAY be able to get inside the casing and fix but otherwise it's a new key I'm afraid.