> My problem with an extension lead is that the modem has to be plugged into the phone point also
You can buy phone extension kits in most DIY and electronics stores and do the job yourself. One setup might be ... a two way splitter downstairs with the phone connected to one of the sockets and an extension lead to the upstairs/PC room connected to the other.
Upstairs you can either wire the end of the extension lead into a wall mounted phone socket
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or else get an in-line coupler so that you can plug the extension lead into one end and the modem into the other end:
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Try to make sure that you use good quality components, make sure everything is firmly connected and not to run the cable longer than necessary or near sources of interference in order to avoid problems with line quality and modem throughput.