We are with Plusnet too, last monthly bill was almost 3 times usual one
After 20 minutes on hold, was told that they agreed, it didn't look right, and it would be investigated.
A few days later I followed up, was asked to send proof that I had paid the August bill (it's on direct debit but duly did screen shot (of last 3 paymeny, just to show how out of the norm it was) and sent.
10 days later, still no action, phoned again (another 20 minutes on hold) turns out that the sudden increase was due to them knitting to take payment for April 2017
Nice chap said that it was their error, and they couldn't legally claim the payment, and they would refund it.
I will check the bank statement and see if they do, can't face another 20 minutes on hold!