Thanks for the kind offer Dillon but I've managed to re-use the old ones again. The new window is fitted & looking lovely.
The background to this was that shortly after buying my van the skylight over the bedroom began to get condensation inside. I raised this with the local dealer during the first hab check & Hymer supplied them with a new window. I never managed to get back to have the new window fitted for over a year by which time Hymer have time barred my claim. I've no problem with this as I should have taken it back sooner. So I had to buy the window for a little less than £60.00. No biggy.
I assumed that there must be a leaky seam between the two halves of the window letting atmospheric moisture in. Well, I was wrong. The leak was caused buy the latch not being sealed onto the plastic inner of the window. The two screw holes securing the latch to the plastic are blind but the there's an additional hole to accomodate the rear of the allen screw that holds the latch together. I'm assuming this is where the moisture has entered.
So, through this hole I've managed to hoover out 90% of the moisture. I've now got a small low pressure airline blowing air into the cavity as the window sits basking in the sun. I'll keep this one as a spare :? I'll probably never need it. I've sealed the holes on the new window with Sikaflex & I'll have to wait a few months to see if this has done the trick.
Many thanks for the responses.