Wheelbarrow01: This looks to be a simple address format issue....
OP was searching for address 18B out of possible 18A, 18B and 18C, however the Chorus system shows addresses of 18, 18B and 18 LF (Lower Flat). I have already confirmed with OP that the target unit is downstairs from 18B, so the 18 LF address seems to match the physical location.
Unfortunately, incorrectly formatted or legacy addresses are prevalent in the Chorus database so it does tend to make things more difficult.
At the end of the day I can see 3 working Chorus-supplied copper connections at 18 - therefore I don't see any issue getting the OP connected. We just need to work out which of the current occupants is soon to vacate - but I believe it will be the person in 18 LF.
Thanks for the reply.
Please check your inbox, it might make things clearer.