Do we need extended warranties for items that may be installed prior to Completion of the Works? Are we protected by the Sale of Goods Act 1979?
There are quite a few unknowns in this question but there are a few points that can be made.
The Sale of Goods Act (or Supply of Goods and Services Act if it was supply and install items) will apply, usually. The application of these acts is extremely fact sensitive so there is no blanket answer I am afraid.
I assume from your question you are thinking of items installed a fair time before Completion of the Whole of the Works and therefore the warranty might expire before the Defects Date. It would always be prudent, where warranties are being given, to have them run at least to or past the Defects Date whenever the item is installed.
What you will most often be looking at is a collateral warranty from the supplier direct to the Employer or eventual owner of the project. The primary benefit of a collateral warranty (also achievable through the Contracts (Rights of Third Party) Act) is to create a direct cause of action between supplier and employer without the claim having to follow the contractual chain through the contractor. The warranty itself is very simple and often just confirms that the contract conditions have been met. So, when you apply that to a normal warranty, you often find that, in reality, the warranty doesn’t give more than the contract did in the first place. A good warranty will dispose of the need to prove a cause of a fault (within a limited time) but the fault is a breach of the underlying contract so the right of action or protection was always there.
There are of course many types of warranties (which should be distinguished from guarantees) so you need to look very carefully at the content, there is not a “one size fits all” answer I am afraid.