High bill due to alleged leak

I received a water bill for just over £1500, which is ridiculously high.

I contacted South Staffs and they said the meter alerted them to a possible leak in Dec ‘18 to Jan ‘19, to which they sent an engineer in Feb. An engineer did come and perform tests, and said he couldn’t find a leak, and said it was only a slight spike/I may have left tap running for a while, which I didn’t.

I received my bill in March which was only slightly higher than usual, for £154, the previous was £147.

They insist there was a leak, using 25+ bath fulls a day for a month. The housing association says there has been no leak or leak repair, but that other tenants had reported higher bills for wrong meter estimates for the previous year.

I logged onto my South Staffs water account to get a copy of the full bill from March to check the water meter reading, as I only kept the first page, but the original one has been deleted by them and replaced with a bill for £1467, which isn’t the one they sent, but strangely the bill date on the letter they have states 03/10/19, 7 months after what it’s meant to be.

They are demanding payment within 7 days. What can I do about this? There 100% wasn’t a leak, and there’s no way I’ve used that much water, I live on my own.