Huw,
Fit the seal to the ali panel and tape in position. Push the glass in sideways so its in the bottom rubber. Work the rubber around the screen with a very, very blunted and polished flat screwdriver till the rubber strip is on the edge of the glass(rubber should be about 90 degrees to the glass and panel), then push the glass back and it should pop in. the appature should be about 4mm larger than the glass the whole way round. use lots of washing up liquid slightly diluted. Ive never had any joy using the string methord as it cuts the rubber.
Jerry