Thanks for the responses.
The PS3 is brand new, 80gb, purchased last weekend.
The DVDs are all new, orginals.
The PS3 has complete functionality otherwise. Games are fine, I've downloaded demos, but not movies yet.
The DVD's auto start but nothing appears on screen. The PS3 menu dissapears and I get a blue screen which is the normal reaction my tv has if there is no signal. The PS3 then becomes unresponsive. Pressing the buttons on the controller have no effect. I have to turn the console off and on again.
I'm connected through a HDMI connection - which works fine with the games.