We too are also going though a similar problem, we started with searching in google and looking at who came top for search queries like "SEO Company London" a bit obvious I know but we figured if they cam e top for a search result like that they might actually be good at it.

However after much pondering and toing a throwing and being general amazed at the cost of such an engagement, we decided to start to create our own SEO strategy. After we got clear on what the strategy was we then became clear on what we needed to do and a project plan started to form.

We then started to look at what we could do ourselves in a short space of time to start the SEO ball rolling.
Simple stuff like meta tags, sitemaps and page content relevancy need lots of work. so we set about doing this first, after all if your content is not right you don't want it on the search engines yet.

We have now since decided to employ our own in house SEO team and believe that is the best way forward especially for getting the biggest bang for your £.

But we did not realise any of this until we had worked out our own SEO strategy and saw the bigger picture. So the point is think hard first about your strategy and find what fits best. Ask some of the SEO companies to come up with a strategy that they would use for your site and see how it compares to your own vision, If they share it then they could be the SEO team for you.

It is a scary world out there with so Many SEO companies after your ££££
However that said If anyone has a good unbiased referral to a hard working white hat SEO agency I would also be interested.

Thank you