Thanks - that's a useful reference. I had in fact cloned the laptop's MAC earlier, without really understanding why, but it seemed then to have no effect and in case got lost in the various resets I carried out.
Interestingly this time it had no effect initially either (router at the end of the roughly 15m long cable from the modem) but then when I retried it with a very short cable it suddenly worked, the status for IPv4 showing the modem-supplied external IP address, the correct time and other indicators to sat connection. Tried the laptop at the end of the long cable from the router (turned off the wireless) and that worked too. However, connecting it to the hub in that room (the next stage for distribution) didn't work either and so having the router beside the modem in the other room wasn't acceptable although, clearly, getting internet connection was a major advance. It looked as if the long cable might be faulty but that I could connect directly to the sat modem and the internet beyond using a direct laptop connection rather belied that.
I read around things a bit more and got conflicting evidence as to straight through or cross-over cables and then found a site which very firmly insisted that the proper Cat5 cabling spec for this sort of general work was T568A rather then the B version I'd used and so I rewired that to the A version and it worked with the router at one end and the modem at the other, some 15m apart.
My one remaining problem is that linking the router to the hub (an ordinary Intel 8-port business hub I've used for years) doesn't work for some reason. This is annoying but not critical as for a couple of the computers I can use wireless and that will leave just enough capacity using the router's four output ports. I use cabling because my wife's office is at least 20m in a direct line and as the house is pretty old the walls are thick and the rooms a bit rambling so wireless would be an expensive nightmare. If the existing cabling doesn't work I'll try rewiring to T568A and see if that brings the same magic as before.
So, many thanks everyone for your welcome help and support. It's been a bit of a bapism of fire for what I'd assumed would be a pretty simple transition but at least I've learned more and at least I've now got up to 22Mbps download compared with sometimes only a few hundred Kbps earlier with ADSL.
Thanks again, MM