On 21 Mar 2004 13:41:04 -0800, in uk.d-i-y (Roly)
strung together this:

A friend has asked me to help her fix some net curtains to a PVA
double glazed bay window.

ITYM PVC.

How should I fasten them ?

As you would with a wooden frame.

There is no wood, just plastic.

My inclination is to drill a hole and screw into it,

You've got it. That should be fine.

but what is
likely to be behind it ?

Most probably, air. At least for a few mm, then probably aluminium.
Plenty of space to screw a net curtain hook into.

Is it strong enough ?

For net curtains, yes.

What about making good the hole afterwards if it proves to be in an
unsuitable position ? - You can's just use filler and paint. It won't
match.

If you drill the holes in the right place there'll be no need to fill
anything in.
--

SJW
A.C.S. Ltd.