The heated bed is now complete! In my last update I was having issues with temperature readings. It turns out that the problem was the ply I used as an insulator. I cut the ply with a route for the nichrome wire to channel in and in the center is the thermistor. Because of the shape, a H shape, the ply had a defect that made the center bend away from the aluminium. As it done so it moved the thermistor away from the aluminium corrupting the reading. Also the nichrome wire was moving away from the bed causing hotspots where it burnt through the kapton tape.
To solve both these problems I used fire cement to fill the grove with the nichrome wire firmly attached. This means that now the heating wire is secure and the ply is held in position.
For the print surface I am using glass but it is borosilicate glass, from RepRapWorld.com. This glass apparently has a lower thermal expansion which, I believe, should reduce the likelihood of shattering due thermal shock. Given the low rates of heating and cooling I don’t think it would be likely occurrence, however I was ordering filament from them and it saves me getting glass cut to the right size and lets be honest “borosilicate” sounds cool!