Failur to respond to a section 13 notice - Assured Tenancy (not AST)

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I have owned the property since July 2009. The property was tenanted when I bought it under an Assured Tenancy Agreement (not AST) which started 1st April 1993. I sent the tenant a section 13 notice on 10th February 2012 proposing a rent increase due to come into effect 1st May 2012, thus giving at least 2 months notice. This was sent via the post office and a proof of posting receipt was obtained at the time sent. Then on 21st April 2012 I sent a reminder letter along with a copy of the notice to the tenant reminding him that he had until 1st May 2012 to either accept the increase or refer it to The Rent Assessment Committee for assesment.
On 1st May 2012 I received a letter from the tenant stating that he hadn’t received the original notice in February but did receive the reminder leter with the copy notice sent 21st April 2012. Due to this he states that he does not feel the rent increase should be accepted by him. This is despite the fact that once he had received the reminder he still had time to refer the notice to The Rent Assessment Committee within the alloted time frame.
My question is, is the rent increase enforceable and if so how do I enforce it?