Thanks so much for your advise. Have just spoken to UW about the back billing principle and their response was 'If you're going to try that one then we'll just make the bill out to look like you've used the electricity in the last 12 months'. They're just a bunch of crooks.

This company gets worse. My elderly mother has just received an electricity bill for £2300 for this month instead of her usual bill of around £80. UW claim that her meter has been under reading since 2014. She lives alone in a relatively small house, with a TV, kettle, microwave and washing machine and purposefully lives frugally as she is a widow and relies on her state pension. She's now wishing herself dead because she can't cope with UW threats to take money that she doesn't have from her bank account or send round collection agents if she can't pay. This company are unscrupulous in their manner of trading and should be investigated.