Andrew wrote:

Velux have quoted about =A3250 to replace the sealed unit. Is it possible=
to
DIY?

If you can get a trestle set up for two to work on, removing a Velux
sash is a doddle. They unclip and lift up and of their hinge. Take it
off and have a look what you can do with it. Then, first thing next
day, take it to wherever for a refurb.

You can take it off by yourself if you are competent. Also you can take
it off from on the roof if you have roof ladders.

How long have you had the Velux?