Not quite the answer you were looking for, but it may well work out cheaper for you to knock out the old windows completely and pop in some nice new uPVC units. If you do it yourself you can get new windows for approx £50 per m2 on the internet. Putting in new uPVC windows is easy (I did my whole house myself, including the doors) and it may look nicer too!
EDIT: I say it may be cheaper, because good polycarb can be as expensive as a sealed double-glazed unit!