Ladbrokes complaint: Odds changed after bet

On Sunday 131122 I placed a bet over the counter. The bet was ed over the counter and placed by cashier. The slip clearly stated the bet odds which were ed by the cashier as it was displayed on the screen.
The following day I went into the shop to collect the winnings and I was informed they will pay me out but not with the odds on the slip but new odds which will mean a deduction of £654.54. The reason for this is due to odds changing which clearly was not the case and if the cashier made a error it shouldn’t be passed over to me. This is clearly unfair and unjust. I placed the bet in good faith and was not informed of odds changed whilst placing the bet, the odds were ed and written on the slip and now that my bet has won the odds have changed and I have to accept odds I never agreed to prior to my bet. Please can you look into this for me and and help me resolve this matter