Landlord/letting agent says we're £200 in rent arrears due to a rent increase we were never notified about – where do we stand? (England)

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Looking for some advice on where we legally stand with this situation.

We rent a small two-bedroom house in Cambridgeshire and have been here since 2020. Throughout that time, we've always paid our rent on time and have never missed a payment.

Last week, I received a phone call from Cornell's, who apparently are part of the same company as Sharman Quinney, the letting agent we rent through.

They told me that we're currently £200 in rent arrears because our rent was increased in April by £50 per month.

The problem is that we were never made aware of the rent increase.

We've gone back through everything we can think of — emails, phone records, letters, messages etc. — and we cannot find any notification whatsoever telling us that the rent was increasing.

I explained this to Sharman Quinney and said that we're obviously happy to pay the new rent of £50 extra per month going forward, but I don't believe it's reasonable to suddenly expect us to pay £200 in alleged arrears when we were never told about the increase.

Their response has been that we apparently would have been contacted two days after the underpayment was received. However, we weren't contacted, and this is the first we've heard anything about the alleged arrears.

Since this happened last week, we've contacted Sharman Quinney several times by both phone and email. We've repeatedly been told that they're "looking into it" or that there is "no update".

We're not refusing to pay the correct rent. We just don't think we should be held responsible for four months of underpayments when we weren't informed that the rent had increased in the first place.

We've been tenants for around six years, have always paid on time, and have never had an issue like this before.

So, where do we actually stand legally?

Does a rent increase have to be formally notified to us in writing?

If they failed to notify us of the increase, can they still claim the £200 as arrears?

Does the fact that we've consistently paid the previous rent amount help our position?

Can they retrospectively increase the rent and demand the difference?

Is there anything specific we should be asking Sharman Quinney/Cornell's for in writing?

Should we pay the £200 "arrears" while they investigate, or wait until they've provided evidence that the increase was properly notified?

We're in England, if that makes a difference.

Any advice from landlords, tenants or anyone familiar with UK tenancy law would be really appreciated.