With twitter, I think it helps to interact with people who are interested in similar things as yourself and might be interested in your product. Seeing as you are inviting people to get their own photographs printed on to mugs and things I would suggest following and interacting with more photographers. Perhaps connect with humorists and cartoonist as well?
Also look out for #DevonHour (Wed between 8-9pm), #SentfromDevon, and in your case #PlymouthHour (Tuesday between 8-9pm)
#ExeterHour is Thursdays from 8-9pm
Naturally I use twitter quite a bit to connect with environmentalists, people who are fans of hedgehogs, butterflies, hedgerows etc as well as printmakers and other creative people.
The hash tag #MadeinDevon already seems to be much used – so how about if this team gets into the habit of using #EtsyDevon when tweeting about our artwork/craft/shops etc so that we can find each other and retweet.
I do notice a spike in views when I tweet a new listing in my shop.