Looking to understand whats caused this dramatic change in our google webmaster account. We did migrate host around June/July - i wouldn't have thought it would've impacted this so greatly?
The only other change is that we've started some paid google ads.
BTW new host is google cloud. 
These general stats might not be telling the whole picture. Using Google Cloud as your new host means that Googlebot has the inside scoop on where all your pages are. So it's possible that the total number of pages indexed on your website has increased (you can check this through Search Console). This in turn means your website is being found for a wider range of search terms.
However, because those might be search terms which aren't especially relevant to you, for many of those searches you're appearing lower in the results. Which drags your overall average position in the results down while also increasing your number of impressions.
Try looking at the impressions and average position for your top performing keywords and see if they have changed with the new host.
