Thanks for your help folks. It is the Mastic Tree, easy to confirm once you have a name to go on. The internet isn't very helpful at suggestions for killing it, everyone seems to want to grow it!. Maybe I should start a nursery as I have hundreds of 'em. It's used for medicines but don't know if it would keep water out.
That reminds me... did you hear about the prostitute who didn't know the difference between vaseline and putty?
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All her windows fell out!