t is easy to fix as my door did the same after our cat pawed the bottom corner. You need to know the profile of the gasket as they come in different types such as bubble, wedge and flipper etc. You then need to measure some key sizes of the gasket such as the bubble height and the foot width/ depth (part that pushes into the frame).
Then go online and google for upvc door gasket or look on ebay and your away from there!
Couple of suupliers I'm aware of are HD Hardware (p+p is £6.50 though for small items but free if order over £60) and Window Supplies Unlimited (who trade on ebay).
Else try at your local glazing company as Shaun suggests -take a small (and I mean small and from top edge so water ingress doesn't become a bigger issue) piece with you to show them.
Good Luck!
Mark R, May 2008
cut about 6 inches off and go to some local window companies to get a match,they should sell you some.hope this helps
