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Having checked your link profile it is very spammy.  25% of your backlinks are for the term "speed dating", 9% for "singles holidays", and thats just a start.

But that is not all.  You have alot of domains sending many links back to your site for the same keyword.  Sitewide links can cause penalties, and will need to be cleaned up.

I recommend using ahrefs \ majestic to find those links and dissavow \ get them to change to no follow etc.

A few examples of sites causing you problems are (but there are many others):

speeddatingcardiff.co.uk

yachtibis.com

whenthemusicstops.com

I am not sure your client fully understands what is meant by this, and indeed I am not sure I fully understand.  However, as I see it, there is an increasing trend for your Google Analytics to report no information when it reports to you, making it harder to get decent view of where your traffic is coming from.

To quote an article: http://searchenginewatch.com/article/2290098/3-Ways-Ecommerce-Websites-Can-Grow-or-Maintain-Organic-Search-Traffic

"Those with their sleeves pushed up working day-to-day on SEO know the reasons why this is happening – searchers who are logged into Google accounts, Firefox users, and most mobile searchers pass "no query" when they click on organic-search links – causing analytics systems to report it without a query."

Having a look at your backlink profile, you lost alot of links around that time, but not referring domains.  I wonder if you deleted a sitewide backlink somehow.  These can give boosts of rankings for a few weeks, but will most likely give you a penalty at some point.

You have quite a few other sitewide backlinks, but they are not that bad (33-40 links each) except pcdynamiceng.com which has 162 according to ahrefs..

I have see alot worse though, so cannot say if this is your problem.

Another thing is the time of year... at the end of the summer people may not look for as many holidays etc.  Have you got metrics going back to last year to compare?.  This may not be relevant, but new sites get a nice boost for the first month or so as Google tries to ascertain whether they are popular \ useful etc.  It could be that this boost has simply faded away (I notice the site has copyright 2013).

All I would recommend at this time, is focus on good quality content and spread that around on your social channels etc.  I would also review all sitewide backlinks, and try to replace those with relevant homepage backlink or editorial backlink.

The way I often approach these things is to "brand out" the homepage, and do two inner pages, one for each location.