If you client has broadband set up using RAKO from a Internet Browser it is really easy and more important it is reliable far better than X10. You can get cheap products from USA or from B&Q but I would not want the risk of having to return to my client every 5 mins to sort problems out. Any RAKO product and a RS232 port connected into a PC will do the job. Obviously that PC would need to be left on all the time. Most modern PC's motherboards can be set to auto restart if there is a power cut so function is always maintained. Who knows your client may like the idea of controlling other stuff later with the possibility of more work.
Do it once, do it right.
Do it once, do it right.