Thank you for your replies;
Ian and Jerevan; the aperture above the lamps is simply an open hole, the only glass is in the frame which sits within the brass around the edge.
Ian, I think you may be on the right track concerning the type of tubes. When I bought them I thought I had done enough homework and purchased the right tubes. They are marked as UVA but perhaps it should be the blue coloured ones you suggest which may be UVB (or something else). I shall go off now, make further enquiries and see how I go.
Vlad; yes, I started with the facial tanning lamp also but thought I would build my own in the hope it would be better! Obviously not the case. It will be interesting to find the exact type of tubes fitted to the Philips tanner. Again, I'll go and check.
Ian, thank you for your offer and if things get seriously difficult I may well take you up.