I just met Paul from Marine Windows, he had a look at my leaky old windows, and rather than suggesting replacing them, he suggested just changing the seals, and filling a few joints with silicon.
If anyone needs window work done on their boat his contact details are:
Paul Hughes
The Boat Yard
Mill Green
Caversham
Reading
RG4 8EX
Tel: 01189482664
Mobile: 07836250219
It says on his card they are:
"Manufacturers of windows, windscreens - hatches
All types of Vehicle windows repaired and replacement service
Diving service available"