When I press the brake pedal on my 05 TD4 the sidelights and the instrument lights illuminate ,has anyone had this problem or got any ideas whats going on?

incorrect bulb, bad earth affecting rear light cluster or if you have towbar electrics there could be water/corrosion in the back of the socket or some other area of the wiring.

Thanks for the reply ,your first suggestion was correct,I changed a stop tail bulb a couple of weeks ago when it blew and everything seemed ok.I checked the bulb this morning it was working but on examining the bulb it was a 21/4W12volt the old bulb was 21/5W12volt I fitted another bulb the same as the original and everything is working as it should.I wouldnt have thought it would make that much difference but obviously it did, thanks...

It is possible that there was a short in the bulb from the stoplight filament to the sidelight filament,
Used to work as a service engineer and found faulty bulbs every now and again.