Now that was a problem I never thought about. I can see it would look a mess and since the next door neighbours have cats who insist on getting onto our window ledges, its pretty likely we'd get scratches. Thanks for the tip off
We have hardwood frames now but the seals are not working so there is a build up of condensation. The guy who came to give us a quote for replacing the units wrinkled his nose and keeping the hardwood frames and was recommending we go for upvc instead (he wasn';t very keen to do the job...