Tenants no longer in occupation, house now being used as unlicensed HMO

Possession - Section 8 and Section 21

My tenants had not paid their rent for two months, and as this was unusual I visited the property. Having tried times to contact them I sent then notice and entered the property to see if they were still living there. I found several beds in each bedroom, also in the lounge and the extension, and the property showed all the signs of now being an HMO. This is a small two bedroom terraced house so is not appropriate for use as an HMO. The tenants who originally moved there has a tenancy agreement from 23 August 2019 and were a man, his partner and their child. I could see no sign of a child's occupation of the property upon my visit today.

My instinct is that they have moved out but have given over the key to somebody else who has immediately moved in other people, is probably collecting rent off them, but is failing to pass it on as well as operating an unlicensed HMO in the process - I have heard of such scams before.

My question is - what notice do I serve on them? I need to clear this house out as soon as possible because such overcrowding is obviously dangerous. What is the best way of ensuring they leave as quickly as possible?