Can an email be a valid Pay Less Notice?

Yes, in principle. A Pay Less Notice lets a payer pay less than the sum otherwise due, and the Construction Act lays down no particular format. Courts judge a notice by what it says rather than its form, so an email or attachment that states the sum the payer thinks is due as at the day of service, shows the workings, and reads to a sensible recipient as a notice of intention to pay less can do the job. In practice the deciding factor is the contract's own notice clause: where it names a method of service or an address, follow it to the letter, because serving by email when the clause requires post is exactly the sort of point that loses an enforcement case. So serve by the method the contract states and copy it by email, labelled identically, keeping the delivery receipt.

A contractor emails a subcontractor a document headed Pay Less Notice, stating £33,000 due today with the deductions itemised. On its content that is a good notice. But the subcontract insists notices go by recorded delivery to the registered office, and this went only by email. At enforcement the subcontractor can argue it was never validly served, and the whole £40,000 Notified Sum could fall due despite the email being spot on. So send it by the recorded method the contract demands and email a copy labelled identically, keeping both the postal proof and the email receipt.