I think you should probably just forget about it and move on. It's not a clear cut case in your favour, so it's not unreasonable for them not to payout.
I lost money recently when a book was vague. I bet on a volleyball match (my home country was playing, otherwise I know nothing about the sport!), and bet on the "full time handicap market". I thought I was betting points handicap, but it was actually set handicap.
The book's rules said that:
"Handicap markets are based on either sets or points (please refer to market title). Over/Under and Odd/Even markets based on points (unless otherwise stated)."
Neither set nor points was specified for the handicap. I assumed it meant points like the Over/Under, Odd/Even rule, and that if they meant set, they'd have specified it.
Needless to say, my team won on a points handicap, lost on a set handicap. I queried why my "winning" bet had been graded losing, only to be told it was a set handicap. I explained that their rules said that the market title would say if it was. They said that it's always a set handicap, unless it says points. I said that's not what the rules say. They said that's what it always is. So, they were vague, I made assumptions, their rules didn't reflect their practice, but I lost my money. I even suggested they just void the bet and correct their vague rules to say "handicap markets are based on sets (unless otherwise stated)." They declined any remedy, and have not corrected their rules.
Perhaps all volleyball bettors (are there many?!!) know this. I didn't, and lost. Anyway, the point of this long story is that sometimes you feel hard done by by a book, and that you "unfairly" lost your money but, like my case, where it's not very clear cut, you can forget about most books ruling in your favour. You, like me, could have been more vigilant - I could have read the rules first, seen the vagueness and had it clarified. I didn't, you didn't, and we both lost out. I lost £170 (my stake, with winnings £350 odd). Hopefully you lost less, and my story makes you feel a bit better about it.