Look up synseal plastics warm core if you want a great product. Brother in laws design. And just because window frames are hollow doesn't make them cheap. Some reet bobbins being talked as usual. Edited because I typed in the original lot he worked for 😳
Highly recommend Nordan (both timber and Aluminium clad timber)
if you want high performance they are a good option
In theory you need to make sure that your new windows comply with current U values. Some local joiner who can knock one up in his workshop probably won't be able to afford to have them tested if he isn't already an accredited manufacturer.
Given the age of this thread, the OP's new windows will be due for replacing soon.
I'm closing this off now, it's a spam magnet. Windows and kitchens, who'd have 'em eh?
