The bill is from an old house for a period when I was living there. But I would pay my rent to the landlord and he would pay the water bills, i.e. water bills where included in the rent.
Apparently Thames Water put my name on the unpaid bill and they got my name from Experian (the credit check company used by my new landlord when I moved to the new place).
Unfortunately I don't have any tenancy agreement for the old place, it was a verbal agreement.
I gave Thames Water my old landlord's mobile number to sort this issue out, but they still call me.