Update. I gave you duff gen there. When I moved the TC closer to the Airport Express (AE) it did indeed then offer me the option of joining the AE and extending the existing wifi network. This seemed good except I want to put the TC somewhere physically out of the way and not be limited to being within close distance of the AE.
I then remembered that I had some TP-Link 'ethernet over the mains' adapters and wondered if that would work, since, the theory seems to be that if the TC is connected to my BT HomeHub modem/router (HH) with a physical cable (and not wifi) it should give me what I want. I set it up but the TC did not seem able to 'see' my BT router.
I then thought - let's see if it actually works connected directly to the back of the HH and powered it up. I got further in so far that the TC offered me the option to replace my existing HH wifi capabilities. I do not want this as obviously my new TC would be underneath the telly next to the BT master socket in clear view.
So, not the product for me and I will return it this morning.