I also got the payment letter and pay by standing order. I gave them a call and it was due to a water rates payment due in June although I didn't receive the letter requesting it – I have received them in the past.
The chap I spoke to was very helpful and said I could pay over the phone (which I did) or increase my standing order to cover the difference between now and the end of the year. I did raise that the tone of the letter was a little harsh but was told they are automated so you get the same letter if you are £10 or £1000 behind.
Although like the rest of you I can't say I was too happy with the letter but can't fault the service I got when I rang them.