Honestly, since I contacted them before the match started, they had two choices.
1- Void the selection (after admitting the odds were wrong)
2- Honour the correct odds of 5.25
The only way I see you having a case here is if you can find proof or a rule that says that Stake is required to make such odds changes. Or if you can get confirmation that the odds provider (Sportsradar) has requested other bookmakers to do so. Otherwise, I think it's a lost cause.