OK...my TW523 two way power line interface arrived in the post this morning. This now allows me to connect my PC to the mains via the serial port (this sounds very dodgy!). Reading the blurb I am assured that the unit is "Opto-Coupled" which presumably means at some stage in the circuit the electrical connection is replaced by light to ensure the mains will never actually reach the serial port...
I had been trying for a long time to find a source for UK compatible X10 units. Suppliers that had previously sold them had stopped due to a problem with CE markings not being on their units. However eventually I found out about Laser Business Systems Ltd who are selling them RIGHT NOW...