In article , says...
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 dothe
job. The reason being for something that they openly admit to having not
costed up.

(Details snipped)

Since you have already received legal and negotiating advice in this
thread I will pass over those bits, apart from one observation:

The legality issue will rest upon how many visits you have had from the
company. The "Subject to Survey" is inevitable and essential when the
company sends a salesman to negotiate a deal with the customer.

Here's how it normally works.
The salesman establishes your requirements for location and style of
window. He measures the apertures and then calculates a price based on
these factors. If you agree the price he draws up the contract which you
sign. Shortly after this meeting the Surveyor will visit. His job is to
verify the technical details of the contract. Are the sizes correct, will
the designs work in the locations specfied, do you need special glass,
can the windows be made to the agreed plan?
If the salesman has done his job properly, then there will be no need for
amendment. HOWEVER........

There are three main areas that catch salesman out:
1. Box sash work
2. Bays
3. Maximum sizes (for a given style)

There are of course others but they are less common.

The surveyor's data is the basis for the fabrication of the windows, not
the salesman's. If the surveyor finds errors, the price is recalculated.
Hence "Subject to Survey".

Now, if you only had one visit, salesman/surveyor combined, (happens with
smaller companies) then your original contract has been surveyed and is
probably valid. If you had two visits, the second was the survey and your
original contract was -- you know the rest.

If I were in your situation (you like the product and the company but not
the new price) I would follow advice given previously and talk them into
a deal they can't refuse. They want your business as much as you want
the windows.

Hope you get what you want.

Ziggur

"S'ils te mordent, mords-les"