1. SBR definitely have big conflicts of interest and can never be fully independent from the bookmakers when pretty much all of their income is from bookmakers' ads and commission but that doesn't mean they won't help the customer just because it is regarding one of their sponsors so if getting nowhere with a bookie then first port of call for me would be SBR.
There are also two other ports of call for dispute middlemen. A guy called Paul Fairhead on Twitter on https://twitter.com/BoycottBetfred
and he is sort of a one man SBR and will take your case on if he feels he can get a result for you.
Also Jimmy Justice on https://twitter.com/gondorffhenry
has a website http://justiceforpunters.org/
that may be able to advise you.
2. If getting nowhere after that and you still feel you are in the right, putting some gentle pressure on through social media can help shake the bookie into gear and if you can google the CEO or somebody high in the company and find out an e-mail address for somebody like that then it can help to contact them.
3. Failing that then the only other option is to follow complaints procedure and go to IBAS which have made some questionable decisions in the past but you do have some hope with them and only step then if no joy is formal legal action through the courts
One thing that is best to do is not to give up if you are in the right and always stay polite with the bookmakers even if they are being obstructive as the staff are more likely to help you if you are being civil with them.
What I would say however is that whenever I see these complaints about bookies not paying up and especially the big boys like Hills and Ladbrokes etc, they aren't going to try and scam you out of money so if they are withholding payment then they feel they have legitimate concerns and as the saying goes, there's no smoke without fire.