rotten woodwork and upvc fitting

Doesn't sound too serious. However, my experience with filler is that it is impossible to get a seamless joint, it seems to pull away as it sets. I reckon enough paint on top could fill any hairline cracks that might otherwise suck in moisture. My gold-plated solution would be to cut out the rot out to a square profile, treat with preservative, allow to dry, then insert a new piece of wood cut to exact size with exterior PVA or some suitable woodglue. I am, however, a glutton for punishment. I would seriously consider cutting out all the rot whatever you do. If too extensive a bodge solution is to use wood hardener, but the area would have to be bone dry, and I expect it ain't. Finally, the reason the wood may have rotted is the presence of the bush, retaining water on the sill, so consider trimming it back.

Andy.

A lot of work and hardly ever successful IME. Car body filler would be a better bet.

I am, however, a

thanks for that, maybe it's a job for next summer ? would have been a good idea to do it before the new window was fitted perhaps.

Ask the fitters what they would recommend.