On more modern ones, it's glazing tape, which is double-sided sticky, and won't come out. It should compress enough to get the external beading out, but then you'll have to cut through it before you can remove the glazing unit. I've also heard the suggestion of using a cheeswire to cut through it. Refitting will require removing all the existing glazing tape debris and cleaning the surface, and fitting new tape.
Finally got round to trying this (there's been a delay while I was doing other things and also I needed to confirm with my neighbour that I should go ahead - before doing anything drastic). Anyway, thank you very much for the instructions - works a treat!
I haven't actually fully removed a unit - only gone as far as proving that it works - as I've just suffered an oohmeback so can't stand upright atm.
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