Dan, I would say that if your reputation preceeds you in the area then the image of the plot would be a secondary concern for many customers.
With the correct renovation, some of the converted garages can actually look very professional, and you retain the benefits of a lower cost plot.
You are in business to make money, and if the current rent/rates make that impossible at the current plot, the only option is to elect another plot. Its just deciding if the plot is appropriate location and size to continue the current level of trading.
On a side note, I know its not a massive ammount in the grand scheme of things, but as an energy broker I may well be able to help on the electricity costs for you. There are several factors that can affect your price on electricty and many are unknown to consumers, and the suppliers wont tell you either. Drop me a PM if you want some free assistance in that area.