OK, making progress (sort of).
Turns out the cable i bought does work, and there IS a signal from the rf output on the stb

I was in a rush last night because i had to go out, so was careless in my tests. I still had the tv tuned to channel 1 , instead of av, and of course with it taking the signal from the stb rather than the incoming cable it was not receiving the analogue terrestrial channels!
I have now modified my set-up to give a short-term solution. Instead of connecting the output from the vcr to my booster (which was causing me to get the video signal upstairs instead of ntl when a video was played downstairs) I have now connected the vcr output to the tv, and the output from the stb to the booster.
Everything downstairs still works

. Because the video is being fed by the incoming cable, and the tv is being fed by the video, I can still get channels 1-5 and can still video one of them whilst watching a cable channel, or vice versa.
Upstairs, because my booster signal is now coming from the stb, I ONLY get whatever channel is viewing on the stb. As long as no-one changes the channel, I can watch my football undisturbed

, and those downstairs will be free to watch any channel from 1 to 5 or a video or a dvd, without affecting my signal. The downside is I no longer get any terrestrial channels upstairs, but I think I will try getting one of those powered electronic set-top aerials with a built-in signal booster. Then if i want to watch 1 to 5 upstairs i just need to unplug the feed from downstairs and plug in the set-top aerial. Better still, is there a simple adapter that will allow me to have both 'aerials' plugged in at once?
Thanks again to all who helped me try to sort this out.