Tenants refuse service of Section 21a

Possession - Section 8 and Section 21

Our tenants ( three young men in their early twenties) caused a fire in our property causing significant damage and have not been able to live there for the last two months whilst we have been sorting out the insurance and repair / refurbishment. The work to refurbish it has just started but we have decided that we do not want them back when it is habitable again and have tried to issue them with a section 21a but they are refusing to arrange to meet with us so that we can serve it to them. They have been living with friends for the past two months but we do not know exactly where. They usually paid rent on 1st of the month so the section 21a requires possession on 31st May 2011. If they refuse to meet with us or tell us where they are living how can we serve the section 21 ? Any suggestions ?