Alan typed

Also tried a new stanley blade, that dosn't work.

Any advice appreciated.

WD40 and leave for a few hours.

We were advised petrol when we had this problem a while ago, on wooden
windows. It is a b*****.

Word of warning for all: masking tape (and Micropore) polymerise in UV light.
Masking tape should never be left on for more than 24 hours damn it...

--
Helen D. Vecht:
Edgware.