In article ,
says...
I'm looking to replace the rubber seals on the opening sections of some nine year old UPVC
windows.

I'm assuming that you are talking about the EPDM seal between the sash
and the fixed frame rather than the gasket which seals the glass into the
sash?
I guess the seals must be almost universal ? or
maybe each manufacturer (thousands of 'em) has their own design ?

Not universal, but not thousands either. There are variations in shape
and size but your replacement need not be identical to the old seal.
You could remove a small sash by simply unscrewing the friction stays
(hinges). Take it to a local fabricator and ask if he could replace the
seals with a new acceptable version. There is a good chance that you will
find someone in your area who could make a match. The replacement is
easy, so you might be able to buy the length you need to do the complete
job at quite low cost.

--
Ziggur

"S'ils te mordent, mords-les"