Advice please re commencing possession proceedings

Rent Arrears

Our tenant is 14 days overdue with the rent. She is not answering her mobile and our e-mails are rejected. We have been advised by our estate agent that we can give 24 hours notice and re-enter and change the locks. Is this true?
If not, we are delivering a letter today giving 24 hours notice of our intention to inspect (under the tenancy agreement) and warning her to pay or we will commence possession proceedings.
Please can you advise whether the short or long procedure is the best one to use? Also, do we have to wait until the rent is 2 months behind, to commence proceedings in the court, or does it just have to be 2 months overdue on the date of our court hearing for possession? Or, is the fact that the rent is overdue, enough to get possession?
Thank you
Luci