Just wondering if this has happened to anyone else...

Two weeks before our double glazing is due to be installed they've just told
us that they can't actually do it for the price that we signed a contract
for 2 months ago. They're now saying that they need an extra £746 to do the
job. The reason being for something that they openly admit to having not
costed up.

Apparently this is a fairly common occurence with double glazing sales. The
company we're dealing claim it's only the second time they've had to do it.

I have sort legal advice and have been told that they're plainly in breach
of contract, but the installer claims that in their T&C's "the price is
subject to a survey". However, the survey was done at the time of signing
the contract.

Although they're very apollogetic and have offered to refund our deposit -
it's all very annoying and a matter of budgets and principles.