hi , if the gasget has shrunk on the inside then be best to pull a bit out and go on ebay ,,, have a look at window gasgets and you will see loads of pictures ,,,
match it up to yours and buy a few meters and then you just wedge it in the same way you pulled it out ,, if stiff spray with a bit of washing up liquid ,, thanks

jason ,,,,,,,, GREEN HOMES WINDOWS

Yes you can. pull out one and take it to any UPVc windows and doors shops,they'll be able to give you the right one and fit it by your self.All you need it a pair of good scissors.
Hope it help.

you can buy the seals from who ever makes your windows. If you cant source them there are companies that sell seals such as http://www.euroseal.co.uk/
They may have what you need

Easily replaced.
Find out the name of the rubber gasket & buy a length from ebay, amazon or similar sales website.
Not difficult to replace new gasket.
Use window cleaning solution to ease the rubber into the groove around the unit.

Regards
All Seasons Group Ltd.
Biggin Hill
Specialists in Orangeries and Conservatories

You should be able to get new rubber gaskets, best get a good window fitter round to advise and price.