What constitutes Rent Arrears?

My tenants have consistently paid their rent late, but they paid.
Now they have stopped paying in full, but have made some part payments.
As the rent is due monthly , for the purpose of serving a section 8 notice,
Can the notice be served :
When the rent is late for the second month i.e one month and 1 day
When the rent is late for the third month i.e two months and 1 day

When a part payment (not a full month) has been received ,
Can the notice be served :
When sum the total rental arrears is a rolling 2 months i.e after 10 weeks because they have paid 2 weeks in the previous 3 months?
I cant seem to find a ‘simple’ answer to this anywhere.
I intend to issue a section 21 notice but in reading the Open Rent contract it states that it can only be issued at the end of the fixed term + 1day, which is contrary to what the notes on the form says?
If the mutual break clause is used what form should be used, and if it went to court (as it probably will) could it get thrown out because of the contract ref to section 21?
After 15 years as a landlord this is the first time this has come up.

Dear Eric4,
I understand your situation, because I’m also in a smellier position now.
Unfortunately in my point of view Section 8 is not friendly for landlords most of the time. (specially with nasty Tenant, They can surprise you at the hearing)
However do you have break clause in your terms? if yes you can use that to issue section 21.
if there is 6 months break clause at the end of six months you can issue section 21.
You can contact few Tenant Eviction companies and get an idea of your current situation.
I use this website; https://www.propertyinvestmentproject.co.uk/blog/a-list-of-tenant-eviction-professionals/

Hi Afam
Thanks for taking the time to reply to my post. Ive taken a look at the link you provided.
On one of the links it stated that the break clause can not be enforced by the court until the end of the term +1 day in the same way as any other Section 21 notice. So it looks like the clause is actually only any use to a tenant who wants to end the tenancy early and not the landlord, or maybe Ive got it wrong.