What concerns me is that they have free-rolled the situation and waited until they have lost before telling you they got the price wrong. For me that's not acceptable. If they had cancelled your bet or offered an adjusted price immediately then fair enough, but once we have played even one more round its not on to change the odds. What about Leicester last year. 5000-1. Imagine if half the way through the season bookies had said, actually this was the wrong price - no shit Sherlock, they just won half their games you are damn right it was the wrong price!!
To highlight my case with the independent arbitrator I would highlight that we are that bad (according to the bookies) that even after we just won a cup competition you can still get 11-1 on us winning a single match tomorrow. I don't think it is beyond the realms of possibility to genuinely think that we were 33-1 to win the cup before the Brighton game - Our form was truly dire having not won in the league in 9 or 10 games, and with 16 teams left the average price would be 15-1, so to end up with 14's doesn't seem right to me.
I would take the complaint as far as you possibly can... you might not end up with 33's but they might meet you in the middle.