My understanding of this, again, is slightly without my immediate range of experience. My understanding is that where there is a dispute, as you describe, they can take that up with the Service Personnel & Veterans Agency.
Also there are pension boards—I may not have the correct terminology there. I know that within areas they have local boards that will help and discuss pension issues. The individual would be able to take up help and advice from one of the service charities to present their case to those boards. I think it's the Ministry of Defence SPVA process that allows people to access appeals in that way.