Looks like you've got the window shut Mick.

When you get home remove the offending hinge and lay it flat on a bench/table. There's a small gizmo inside that locks into notches in the hinge channel and when you lift up the gizmo moves out of the way. Obviously yours is stuck in the lock position.

Easier to play with it and get is released on a workbench than in a field.

Yes, not the same hinge but you'll get my drift:-

Hi Mick. I managed to replace the two rear window hinges on our Eriba a few years ago and had no difficulty in removing the old ones. Can't remember what I did and currently can't find any info about it. I'll keep looking.
It must have been straightforward as I've no longer got the patience to deal with any bovver.
Randa

The arm has a long sliding dovetail housing on the back of it. On the widow is fixed a plastic block with the matching profile. To remove one from the other you need to find the 2mm hole, which I cannot remember is through which piece. Into this poke a skewer or possibly cocktail stick and the two will slide apart.
Each year I give these ratchet stays a good spray of PTFE, as I've also suffered the same problem in the past.

Colin

How Annoying. I can give the exact date that I changed the hinges (27th July 2019) and a few pics of the broken gizmo but I' can't find and reference as to how I removed them and fixed the new ones.

For what it's worth these a re a few of the photos I took at the time.

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Hold on, looking at your photo again Mick, you can take the whole hinge off including the "locking lever".

Randa

I'll try and explain it another way. The hole you are looking for is not along the length of the aluminium stay.
One end of the sliding arm is screwed to the aluminium trim around the window, which you have released, the other is a hinged black plastic lump attached to the window. This fitting is a plastic mount bonded? to the widow, onto which slides the plastic lump on the end of the sliding arm. To the best of my memory from experiencing the same problem in 2012, the 2mm hole you are looking for is in this plastic lump. When you push a small meat skewer into the hole it pushes down a plastic clip and allows the fitting to be slid off the mount.

Colin

Be careful when ordering a new pair of hinges. There are two types with little difference between them. I can't quite remember but the type I ordered had a slightly different handle fixing so I needed to do a bit of plastic removal and re-drilling to get them to fit.

The ones I didn't order would have been perfect.

Randa

Don't forget you can get the same sizes in the sliding tube type with locking thumbscrew that are infinitely variable. I used this type on the pair I replaced.

Colin