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Possession - Section 8 and Section 21

I have rented out our house on an AST agreement with 4 tenants.
For three of the tenants the AST started on the 10 April 2013 for one year and the other tenant has a six month AST which started on 10th Oct.
The AST is due to finish on the 10 April 2014 for all tenants.

If we wish to take possession of the property in April 2014 do I have to serve them a S21 (1)B form and state the date as after “10 April 2014”.
They should pay the rent on this day but at times they pay a few days late as they have not complied with our request to pay by standing order / direct debit.
I know I have to give them 2 months notice so delivering the S21 would have to be done before 9th Feb 2014 but would it be best to give them more notice than this?
Do I have to give each individual an S21 in Person or would it be enough to get one tenant to sign and leave a copy for the others and follow up with an email copy to all?
We do not have a problem with the tenants but need to do repairs to the roof and bathroom which we cannot do with tenants in situ.
Thanks for your help
J Burt