What if they turn around and say they did not receive the S21?

Hi Surjit

You're in danger of confusing two issues here - 1 the s231 notice and 2 the rent arrears. If there are arrears then you need to get the agent to serve a different notice.

As for the s21, firstly check and see what the tenancy agreement says about serving notice. First class post is not necessarily wrong, unless the agreement says something different. There is case law on the service of notice and your agent should be familar with this. The ideal is personal service with the notice handed to the tenant and witnessed (maybe even photographed), but sometimes that is not possible. You need to check what records the agent has (e.g. a post book). Unfortunately tenants rarely sign for Recorded Delivery letters (especially if they are expected) so 1st class post can sometimes be the only option.

I suggest you ask your agent to verify delivery and see what they say.

Sorry to say that if your tenant says they did not receive it, and your agent did not get "proof of postage", I would suggest serving it again and not wasting your money applying to the court as, it is highly likely the judge will rule in favour of the tenant as there is no evidence of the notice being posted.

I also suggest that if the tenant is now two months in arrears or more that you serve a section 8 notice.

Serve a copy by first class post, get proof of posting from the Post Office and also serve a copy by email cc:ing a copy to yourself (or via your agents). If you have the tenant on Whatsapp, serve a notice through that media.

That way you will have ample evidence of receipt. If the tenant chooses not to read the messages, they can't claim they weren't notified.

Hi Surjit
Are you aware of the new rules because of COVID? Sounds like you are not. Did your agents give 3 months notice? Did they advise you there is a moratorium on commencing proceedings and you cannot take action before 23 August? Tenants will know this and may be reluctant to move when they don't have to.
You say: "What’s my next step? I’m a first-time landlord and want to move back into my property, but just don’t know how to get them out." If you are not confident your agents can deal with the matter, instruct a solicitor. The solicitor can also check other legal requirements that are a precondition to a s21 notice have been met.
You go on to say: "They are paying £1100 a month when they should be paying £1750 a month!" This is ambiguous. Do you mean that the market rent is £1750 but their tenancy says £1100 or that the tenancy says £1750 but they are only paying £1100? If they are in default you need to assess whether they can pay but won't or cannot afford to pay, e.g. because of redundancy. If the latter, your best option is to get them out quickly and write off arrears.

First class post is the correct way to serve notice as long as you get a proof of postage. If the tenancy agreement say by way of letter then that's what you do. If you end up going to court and produce the proof of postage the judge will assume it has been served and received.

I don't think you need worry about whether your tenants will say they never received the notice by post. As long as first class post is provided for as a means of service in your tenancy agreement, (or not ruled out), service will be deemed to have occurred. You do need to ensure that all the other prerequisites have been adhered to, to the letter, to ensure your notice is valid though.
As Ian says, you haven't really been clear about the rent point that you make and actually that is really not relevant to getting them out. As Ian also says, the notice period has increased to 3 months due to covid (which I imagine your agents are aware of), and you will suffer from the backlog in the court process. This is all really uncharted ground as covid is a new situation and none of us knows how long the court process will take. I would suggest you get professional advice from an eviction specialist if time is of the essence for you. I would also look for alternative accommodation for yourself and your family whilst you navigate this process as you are likely to need it.