Hi Gareth,
First of all your issue might not be Penguin related -> http://screencast.com/t/ocuuKXoKUz0v
Just because the drop "happen" after the last penguin update and also because your link profile dosen't look like a penguin candidate.
Also you should check and see if you have some manual action (maybe you got hacked or something) - now you can see that in Web master Tools - hope you the account already in place.
That being said I would't invest a dime in SEO at this stage but I would definitely invest in redoing the site - there is so much room to make it more friendly and attractive - starting with the font size and the way the products are featured and presented, images as far as resolution, quality, navigation flow and many more.
On the SEO side just make sure you have the right thing sin place - and again, no need for SEO "experts" sending advice and invoices - just take care of the normal stuff like Title tags, and descriptions and H1 that are all working as one. Target a keyword per page and push some informative descriptions and reviews and images, videos on those pages.
Just my 2c.
