Gosh! Thank you for such swift responses! It's nice to have immediate access to people who have a clue what's going on!
I forgot to mention (I didn't want to over-complicate the arrangement!). Our ACTUAL installation consists of the following:
1) NTL wallbox supplying cable modem feed. Unsplit.
2) NTL wallbox supplying TV feed. Unsplit.
3) NTL wallbox supplying upstairs TV's. Split x2.
The upstairs wallbox (3) has been split to feed TV's in 2x bedrooms. One of those is actually a PC in my son's room with a TV card. Seeing great picture quality so I have no reason to believe that the signal is a problem.
I imagine that when (3) was added (about 3 months ago) the NTL engineer did whatever he needed to in order to bring up the signal on that additional pace box. Only at that time there was no 2nd TV connection, he "tuned" the connection for one PC on an extra pace box.
What I'm looking to do is feed MY PC with a TV feed from (2), or ideally (1).
Have looked at supplying a signal from an external aerial. The result was
this (re-arrange letters).
A