We are in a dispute worth £14,500, with a builder regarding
substandard work. My partner insisted on bringing a Quantity Surveyor
in after much of the unsatisfactory work had been rectified. 
Unsuprisingly, the conclusion in the report was that the work had been
carried out to a reasonable standard. Are quantity Surveyor's reports
confidential and how much weight do they carry in a court of law

Who paid the QS's bill?

If you or your partner then you are entitled to see the full report.

If the case should go to Court, the QS's report will carry a lot of clout.

Regardless, it's not a good idea to go to court!

Avoid it like the plague ...

I know of several such disputes where the plaintives have been very
disappointed and disadvantaged by the outcomes.

I like what the previous person said.

make sure the QS was independent and if you are not happy get a second
opinion from another QS that is totally of your choice.

--Keystroke-ga