Not strictly DIY, but I've been here to the NG for advice before.... Having d-glazing fitted now...... what a nightmare. Wish we hadn't started; several windows (esp 1970s crappy timber replacements) in this 1920s house were rotten so something had to be done, and none of 3 joiner-men we had round to quote ever came back with a quote. In view of this, some HAD to be replaced, only the upvc companies would take on the job. Would'nt have believed it would be so stressful, so depressing - wrond pattern of obcure glass fitted in several - had to be re-done, wrong colour handles on a door - had to be redone, wrong frame size - had to be redone (3 times till they got it right), damaged handles - will have to be replaced - broken kitchen tile round window opening - "hope you've got some spares sir".
Thought I'd share my nightmare with you, but in absence of joiners willing to do the work what alternative is there..... :-(