They tried that with us a few weeks ago. I made clear in no incertain terms I wouldn't stand for it and they backed down. I made clear the fact that no other company had tried to do that in 30 years of buying electricity to us including Opus and that if they didn't back down then we would leave as they'd breached the contract. They didn't argue that point. But also in our favour was the fact that they'd mentioned higher pass-through costs being known by them (in their letter to bump prices up) about a month before they quoted and we signed, so they had no leg to stand on as they knew everything prior.The secret's buried in the contract terms. When you sign up to the deal, the small print tells you that if the company's costs increase, they can increase their charges. So the 'fixed price' that you're on is actually an illusion. Waste of time, really.
The industry gives this an important-sounding name: 'pass-through' costs. So if your supplier has a supplier and that supplier raises his prices, your supplier 'passes through' that cost to you. It's doublespeak for 'we'll jack up our charges whenever we feel like it'.
That said, Opus is the only company that's done it to me within the period of a contract. And they dropped it when I kicked up.
Curiously, we have two meters with them but they only tried the scam on one meter.