Should I take action to forfeit lease

Rent Arrears

We have 2 tenants in our property occupying under a periodic tenancy (fixed term having expired). One of the tenants has received a job offer abroad and wants to end the tenancy, the other has found another flat mate and wants a new tenancy of this property. We served a s.21 notice to end the current tenancy and the notice expires on 9th May 2010. There are 2 guarantors. The rent has been persistently late throughout the tenancy and now the outgoing tenant's guarantor (who usually pays the rent) is refusing to pay it and hasn't done so,giving an unfounded claim that our agents who deposited the rent deposit didn't give the Prescribed Info. which they clearly did.
The new tenants want to start the new tenancy asap and the outgoing tenant has now gone abroad but left a lot of his belongings including his bed behind. Should I take action to forfeit and if so how long does it take or wait till 9th May?