Thanks for the update, I have just called the UK Information Commissioners Office (ICO) and they confirmed that there is no Legal minimum size, the only requirement is that it must be visible to the public.
As my property is a private residence I want to stick to the letter of the law and...
Does anyone know if there is a minimum legal dimension for UK CCTV Signage and what sections of GDPR and Data Protection act the dimensions are defined.