timbo wrote:

I must admit I was expecting to be told NOT to make
holes in the UPVC!

Can you advise what length screws to go for if I'm
going into the UPVC? How thick is the UPVC, I've
always imagined it's very thin with lots of air behind
it.

It's only 2-3mm thick, but there might be more than one layer of plastic
which you can drill through. Sometimes there's a metal frame hidden
somewhere inside the upvc - you can drill and screw through as many layers
as you like, or you can accept it as a limit. Stopping before you see
daylight is considered good form.

Re twin thread woodscrews, are they easy to get at say
B&Q? (I'm only a casual DIY-er...)

Dead easy - look at the depth of your blind fittings, and choose a screw
length that will stick out for 25mm or so beyond that.

Buy a small pack of self-tapping screws if you want something really simple,
or a pack of twinthread woodscrews (threaded for full length of screw) if
you want to keep the rest for future odd-jobs. No. 6 size will be fine if
there's several screws going in at each end, probably No.8 if there's only
one at each end.

Use a 2mm drill, or a hand bradawl, or any other sharp pointy thing, to make
a pilot hole where the screw is going to go. Needs to be about half the
width of the screws you're going to use. This will reduce any problems with
slipping, and helps to double check measurements.

If you're not sure you've lined it up correctly, just move it - the
redundant screw hole is likely to be hidden under the fitting anyway, but if
not a dab of paint or tippex will cover it.

..