Your on page 15 for google.co.uk
is there a specific page you are targeting?or is it just your home page? either way have you updated the page in a while? i ask this because i would think that the keywords you are after would have a lot of competition and if your site doesn't seem as active as the others that could be one way for you to lose some ranking.
another thing could be what the others are doing. if they are just more active in getting high quality inbound links that you then that could be another problem. are there many different sites in the pages before your's or is it a small set of sites?
google is changing its algo, so, that is not a big surprise to me. many webmasters out there lost most of their ranking overnight. Just keep up with google's algo, see what's work and what's not to do in terms of linkbuilding, change your strategy and you should be fine.