Proof that person has received section 21 eviction notice

[Langanby]

I want to issue section 21 eviction notices to 3 tenants. I am concerned that 1 of the tenants may deny receiving the section 21 as they have been a bit awkward. I read online that the best way to prove this is to have a witness to them signing a document saying they have signed the section 21, but I have also read that the tenant does not have to sign witnessed document even if I say they do. Other advice is to use signed delivery, but of course there is no compulsion to receive a signed for item. Please advise