Okay, this post is to 'clean up' and bring to a close this thread.
I've been involved since post 39, when the thread became involved in an issue tangential to the initial one.
Thanks to sparkymarka, I was able to isolate the problem with the BPT system (described in post 41) to a wire going under the drive to the gate. It was impossible to replace this wire without digging up the drive, which I don't want to do.
I then started thinking about replacing the whole system with a wifi-based one. The starting cost of these would be in the region of £100.
I then realised that, (at least for now), there is only ONE of the wires which is defective: the one which "rings the bell" in the hallway. The other components (ie the ability to hear and talk through the intercom) still work. So all I had to do was get a wireless doorbell. I sourced one of these for £6.49 from Screwfix. It is working fine.