Section 13 no response from tenant

Rent Arrears

In August, we issued a section 13 rent increase to our tenant. We hand delivered the letter with photos as proof, given one months notice. Didn't hear from the tenant so contacted her when she paid her normal rent in September. She denied all knowledge of the section 13 being received so I emailed a copy. No response from tenant again ( she gets my emails ) rent is now due and I know she won't pay. When we mentioned at the beginning of the year that the rent would increase, she contacted environmental health has she wanted a brand new patio ( noghing wrong with the patio). So far this year she has had :

New fence with concrete posts

New oven

New extractor

New smoke alarms throughout

New carpet and underlay

All windows fixed

New front door

New shower

We always use good quality work and the house is nice!

She told me she won't pay a higher rent as the bathroom floor needs to be replaced ( it does) but this was booked in for the summer when she was on holiday. She then messaged us to say that if we do the work whilst she is away she will call the police!

Her current rent is £925 per month. She is in arrears of around £1300 as she decided to pay £900per month and paid £450 less one month. Her rent has only increased by £125 in 10 years. We are proposing her new rent to be £1015.00. She won't co-operate to have the work done but then won't pay the rent because of this. She also contacted our plumber direct to say we told her she can arrange a new boiler, however she has never spoke to us about an issue with the boiler. Where do I stand?