On Sunday, 22 July 2018 18:10:32 UTC+1, Mathew Newton wrote:
I want to remove a UPVC window

[...]

Any ideas how this window is secured in place? Incidentally, the inside of the frame shows no obvious fixings passing through to anything either.

I finally got round to the removing the window (and a set of French doors for that matter) and can confirm that they were held in place by a combination of 'conventional' fixing cleats (top and bottom) and a proprietary cavity closer with a full height clip on the sides:

http://www.newtonnet.co.uk/permanent/loungewindow5.jpg
http://www.newtonnet.co.uk/permanent/loungewindow6.jpg

The clip was easy to remove once I knew it'd come out, the cleats were a bit more difficult as the top ones appeared to spot welded (possibly rivetted?) to the lintel above and so I ended up hacksawing them free.

Thanks everyone for the inputs - they were most helpful in getting my head around what possibilities I was facing.