Power loss with lights on !!!!!! ??????

I have just had a miserable time in Port Talbot, for many reasons, but the worst has been this new, bizarre problem with my tdi Freelander ......... whenever I turn the side / head lights on there is an immediate loss of power. It is almost as if the fuel pump stops working or the engine is denied any air. When the lights are tuned off, there continues to be power loss for a couple of seconds and then power returns and off we go !

It drives fine with the lights off.

Does anybody have any ideas what the problem is or how best to try and find out what is causing this ?

Your help would be most appreciated!

Karcus

RevJames has just suggested that the battery may be on the way out - the extra demand when the lights go on is too much for the alternator ........ sounds plausible. I wonder though if the 'Battery' light would be coming on on the dash if this was the case ?

Karcus