Let me throw in my 2 cents.
- is there any added SEO benefit to trust pilot?
No, there is not a "boost" in organic rankings that we know of. However, the latest Google Quality Rater guidelines does look for reviews. You can find all kinds of info here. I read that whole guideline and it sounds like Google will soon start incorporating 3rd party reviews into their algorithms soon if they haven't already done so.
- would our star rating show up on our organic listings as we don't run any adwords campaigns (we are happy with our ranking in the organic listings).
You are talking about Merchant Reviews (or Seller Reviews) vs Product Reviews. These are two different things. One is as a seller as a whole like "fast shipping", "customer service sucks", "awesome company", etc. The product reviews are specifically for a product (which show up in Google Shopping and in Organic.
- what happens when google and trust pilot fall out and bin the partnership?
SOL.
Since you're asking if Trust Pilot is worth it (you have to pay $4800 upfront for the year), look at the alternative Shopper Approved ($39/mo month to month 60 day free trial).
BOTH are great companies.

