Go for it Jos 1 but please remember that new windows now have to conform with shed loads of minium specs to comply with the new building regs. Check with your local building inspector before you start making them to see if they comply. Also, you will need to get a certificate from the building reg office to comply with part L of the new regs, this should cost about �55.oo. This is in case you sell your house at a later date and is mandatory.
Its a bit of a mind field now so take care.
Johnny edge.