I can see daylight between my upvc window and frame

[Julio Silveira]

10-15 years old and I assume you haven't always had these gaps?

For the sash's to be this far out now your bay must have dropped at least an inch, Quite possibly more.
I would say you have a serious structural issue.

There is a good chance that the foundations are subsiding.
Bay windows quite often didn't have very good footings but it could be down to cracked drains or nearby trees too.

It could still be a problem with the structural posts on the windows. Inadequate baring at the top, No baypole jacks, It could just be that the timber frame between the 2 bays is rotting.

You could contact your house insurance people but once they are involved your premiums will likely go way up and it will come up in any search's done on the house if you ever try to sell so its worth trying to at least work out the cause before getting them in.

Have you got these gaps on both the upstairs and downstairs windows?
So post lots of pics from the outside.
Some pics of the large sash's on the inside of the downstairs bay (to see if they are out of square too).

I have only lived here 1 year so no idea how long it has been there.

Downstairs bay window seems ok. All windows sealing properly. Maybe not 100% squared but not as bad as upstairs. See attached pictures.

I think if there was a structural problem larger cracks would be visible.

 

[Julio Silveira]

I have only lived here 1 year so no idea how long it has been there.

Downstairs bay window seems ok. All windows sealing properly. Maybe not 100% squared but not as bad as upstairs. See attached pictures.

I think if there was a structural problem larger cracks would be visible.

Some more pictures

 

[Gazman16]

Hard to tell from the pics but nothing super obvious.
Only thing I can possibly spot is the PVC cills do seem to look compressed and bent?
This could indicate no baypole jacks used (quite common back then) so the weight may have crushed the cills a bit allowing movement.

You could pull off the external corner cover caps to take a look.

Ideally you could do with a report from a structural engineer to confirm. Probably cost about £300-500 but would put your mind at rest and if it turns out to be something major you could probably claim that cost back when you put in an insurance claim for the repairs (assuming of course its covered)