Thanks for all the replies. Having taken on board that if I use a longer cable I need cat5 (and having done the obligatory Internet searching) is this what I need?
It is described as a "CAT5e RJ45 - RJ45 (Moulded) Patch cable, length 20Metres", Price (inc VAT) £7.17 + P&P.
Re the wireless route, can anyone supply any useful links so that I can start finding out what it's all about?
PS I'm glad I found out that changing the length of the co-ax cable might cause problems before I actually tried it!