On Sat, 20 Jan 2018 04:35:45 -0800 (PST), David
wrote:

Hi

On a couple of our windows, a small (1 mm+) gap has appeared between the window and the interior plastic sill. It only runs for 250mm or so.

Is this a horizontal gap?

Anybody got an idea of w suitable filler for this?

When just filling (rather than bonding) I've used 'Decorators filler'
because it can be wiped with a damp cloth.

I'd like to avoid silicone if poss.

Me too, unless running a bead the entire length etc.

It there suitable white plastic filler that dries hard?

Because uPVC moves all over the place then unless you are sticking two
bits together permanently then you might be better off with something
that maintains some flexibility?

Some in-laws had two bay windows fitted and log short, they made a
pigs ear of it.

The inner sills were just stood on towers of packing pieces and
siliconed in place. So remedial silicone was ever going to fix that so
I ripped it all out, built inner and outer wall back up to the right
height and reset the sill back on solidly.

The issue was first discovered when they tried to place a pot plant on
there and it sunk 1/2"! ;-(

Cheers, T i m