For domestic supplies, they cannot bill you for more than you've used in the last year. You should contact one of the following to see if that applies to business supplies

http://www.consumerdirect.gov.uk/

http://www.consumerfocus.org.uk/

Once you've established what you owe, you can discuuss paying in installments.

You should check that the backdated bill is charging the units at the same rate as your agreement because they sometimes use silly "out of contract" rates for new customers who don't tell them when they start using the supply.

i joined sottish power about 5yrs ago having a business with living accomadation and advised to pay 120 pounds pcm
standing order and any adjustments would be made however in this time we have never recieved any bills
and assumed everything was correct all meters have been read regullary but now they tell me they have never charged
us for our 3phase supply and are now back charging nearly 30,000 pounds could you advise our options