Section 3 of that Act, together with the Consumer Protection from Unfair Trading Regulations 2008 and the Estate Agents (Specified Offences) (No. 2) Order 1991.
The 2008 Regulations are about protecting the public from unscrupulous trading practices in general, including various examples of misleading by omitting information. The 1991 Order, as amended, makes that a designated offence for Section 3(a)(i)(iii) of the 1979 Act.
That means that the "lead enforcement authority" could impose various professional sanctions. That authority is the National Trading Standards Estate and Letting Agency Team, housed (for obscure local government reasons) within Powys County Council but with a nationwide remit. In their guidance for estate agents, linked from https://www.propertymark.co.uk/resource/ntselat-guidance-on-property-sales-and-lettings.html, they list not passing on offers as an example of a misleading omission.
(But the seller may have given them instructions not to bother passing on non-competitive offers, for example, which would be OK.)