I think Plusnet have pulled a fly one here

[#1]

My contract was renewed on March 5th I was sen an email with the new contract. which stated "

The price for broadband, line rental, call plans and call charges will be increased on or after 1st March every year from March 2022 by the Consumer Price Index rate of inflation published in January of that year plus 3.9%.

I thought o good I have missed the price increase  but I got another email later on that stated

On 31st March 2023 the price for your Plusnet services will be increasing by £5.84 a month. This is in line with your current terms and conditions, which are linked to inflation.

Am I wrong in assuming that on or after the first means a few working days and not the whole month

[#2]

Personally, I would have the very strong view that a contract started on 5th March would not be subject to a price increase associated with 1st March  - unless that was made very explicit in the sign up process (in which case I would almost certainly have refused it).

[#3]

I did complain to customer services who said that  as I agreed to the new contract in February that the price rise applies as I was incorrect in assuming that after 1st March meant within a few working days  not 30 days