Evicting Tenant - Section 21 vs Section 8

[dazmanultra]

A tenant seems to have made themselves very scarce, and is currently 3 weeks late on their rent. The managing agent has had no luck in getting in touch with them or their guarantor. As such, I'm thinking that perhaps it's time to start looking at removing them. It would be one thing to have an excuse - perhaps they've been paid late, or something else has come up, but to be uncontactable to me seems shady. In just over a week they'll be two months in arrears so I can potentially look at evicting them under Section 8 (non payment of rent), or alternatively I can start proceedings now with a Section 21 notice now that 6 months have expired on the tenancy. The reason I would be leaning towards section 21 is that this is the second time in 6 months the rent has been late, so I think that it's probably best for all parties to look at alternative arrangements. With section 8, is it possible they could delay and delay and delay and then make a payment of some of the rent at the last moment to stave off the court action?

Obviously I'll need to take some professional legal advice to ensure everything is done to the letter but some general pointers or people who have experienced this would be helpful.