Yes, a pair of long tweezers extricated the double red cable from the others and it slid about inside the cable tie holding the bundle enough to pull the little plug of the cable connector from the socket. So I got it out.
Then I had what thought was a bright idea of threading the cable from the new sensor into place and connecting it before I had to take up the slack and screw the bolted sensor end into the boiler. Of course that meant that as I turned the threaded bolt into place the wire twisted. Doh. Once i undid it and re-did it it went together fine, albeit not cable tied into place.
Now, no error code and correct temperatures. Thanks @DavecUK for the help. 👍