[grbrown]

It involves removing the tank (draining contents first) and unravelling the old cable, then routing the new one along a similar path. Can be a challenge, as sometimes these things go via strange routes! If the path looks convoluted, disconnect both ends, tie/tape the new one to the old one and gently guide through. On the other hand it could be bl*****g obvious and much easier than that!

Halfords sell a plastic self-sealing coupling you can put in the connecting pipe at the front of the tank. You will have to drain the tank this time, but next time simply disconnect the connector and hey presto!