Pinot Grigio wrote:
A large double-glazed window has developed a crack from one corner about 12
inches toward the centre of the window.

The heat and cold is making the crack grow longer.

What can I do to stop the crack travelling further. Cellotape probably
won't work because of condensation.

Gently rub it over with meths on a tissue, gently dry it,
stick some tape over it just in case, change it when you
can, be careful when handling the pane that the glass
doesn't tear when you're not expecting it. You've got an
approximately 0 chance of drilling it or anything like
that - it's knackered, tape it up for safety and leave
it alone!