No, there is no legal requirement to carry an ELT on European light aircraft - but of course you can if you wish.
As of the 15th March 2007 there is now a legal requirement to carry an ELT if you are flying over water more that ten minutes cruising time away from land. Having spoken to the man from the CAA yesterday, to comply with the legislation the ELT must conform to CAA/EASA standards. He stated that there are no portable ELT's that meet these standards. When I queried why this new legislation hadn't been more widely promulgated he informed me that it had been out for consultation for two years. Quite who they consulted with I am not sure. For those looking for chapter and verse on this the ANO has been amended but CAP 393 is still going through the publishing stage.