Bill backdated to 2019

Bills22

I left my property last year thinking all my bills were paid.  I contacted Thames Water who said I owed £219 which I paid.  Now 6 months later they have sent a bill backdated to 2019 until my leaving date saying I owe them £150+.  The meter was covered in dirt so I could not check any readings.  I had lived in the property 20 years and was never behind.  The increase does not match the amounts shown on the document on this website.
Can Thames water suddenly increase bills and back date them like this?

pochase

When you contacted them telling them you are moving, you did not give them a meter reading?

I assume somebody managed to clean the meter so they could give a reading when they moved in. 

Did you receive a final bill, or did they just an estimate that you paid? Without meter reading it is unlikely that they would give you a final bill.