In article ,
Michael D wrote:
If mastik is still the best thing - any suggestions about what is the
best kind for this kind of job (given that I need it to be robust and
cope with a fairly small gap)?

Silicone sealant is the standard thing with double glazing etc.

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*Being healthy is merely the slowest possible rate at which one can die.

Dave Plowman London SW 12
RIP Acorn