I am having a glass cooker splashback put which has mirror screws fixings.
Which would you recommend ..a chrome cap or a plastic one,...thinking about
steam from the cooking.
I can't see that steam from the cooking would damage the screw caps, whatever material they are made of. Mirror screws are designed to be used in damp, steamy atmospheres, anyway, and the chrome plating will stand up to that kind of use. My preference: chrome caps.