Just got a huge final bill from my gas/elec provider as switching to another?

Go read the meter and check your on-line bills ... (they can make mistakes ...)

No doubt your existing supplier sucked you in with 'lower payments' .. which means you underpay for about a year and then get hit with a massive bill when you decide to switch again ...

... but don't worry, your NEW supplier will be doing EXACTLY the same (since that's the only way they can come in lower than your current underpayments) ...

If you visit the Citizens' Advice Bureau they will check the validity of the bill and also negotiate staged payments on your behalf. I had a bill for over £1,000 when I changed over once but fortunately for me it was a mistake and I didn't have to pay it. Check that the reading you gave your old provider is the reading the new provider received.

I've had a similar experience. Did you pay by direct debit? Maybe they had set the payments too low or something. With me it was a prepayment meter. I changed suppliers and got a bill for nearly £200. I couldn't understand it, surely the point of a prepayment meter is that you never owe them anything. They told me the meter was not picking up signals from their main computer or something equally baffling and had not updated itself last time the prices went up. I argued the point that it was their problem, I'd paid up in good faith. All to no avail, I had to pay, I was fuming.

Check the readings and make sure they are correct.