how can i get an independent assessment on an insurance policy I believe was missold to me, have gone as far as the FOS ?

What type of insurance were you missold? Payment protection? If so then there is a goal on this website that details the process that you should go through:

https://my.money.co.uk/goals/browse-goals/goal.htm#goal=34

Why do you suspect it was missold? Did they state it was compulsory or say it offered something that wasn't covered? What have the FOS said? I presume you have written them and the insurance provider a letter regarding the issue?

The FOS are normally the last stop for an independent assessment, but it depends what they have replied to you.

I believe the FOS only step in if your complaint isn't resolved within 8 weeks, so you need to give the company who sold you the insurance policy 8 weeks to resolve it. As TheBandit rightly says, what you could do next depends on what the FOS have said to you in regard to your complaint, if the 8 weeks is up without a resolution.