Lets just say I worked for them for 29 years and wouldn't insure with them if they were the last company on earth.!!
Threaten them with Insurance Ombudsman and you will get satisfaction,they are terrified of them getting involved.!!
Good Luck.
The FOS is not the answer to everything - I would excercise their complaint's procedure in the first instance - get them to explain in writing the reasons why they are not paying out.
If you feel you have grounds to dispute this then escalate accordingly. Obviously if you get no where then yes, go to the FOS, but they may refer you back to the insurance company if you have not recieved their final response.