Thanks for your help, the cable has now been extended and I have Internet access like before.
One trip to my local Maplin shop, sorted it out, I got the 20 metres of cable and the connectors without any problem. It was almost too simple a task to do, even crawling around underneath the floorboards didn't seem like any difficult challenge (luckily I had my trusty mobile phone as a torch).
Note: I had previously wired my house with an Ethernet network, which helped my confidence.
