MartynG Hang on a minute! You gave us your postcode and I put it into the coverage Checker and the Network Status Checker to see what they said - of course I wouldn't know where you re otherwise. The checkers work on postcode - if you are next to a high-rise building full of steel or similar, that would affect your quality of signal.
Also, it may have escaped your notice, but this is the Community Forum and we are customers who see if we can be of help - we don't work for smarty and so cannot sort the service for you.
I pointed out that they say that Three (on whose network Smarty runs) are working on the network in your area.
Unfortunately, you may have no choice other than to move to a better network for you. Smarty obviously know that the area needs work and are getting it done.
Finally, if you are running a business as your final line suggests, then Smarty specifically states in T&C's that the service is for non-commercial use only.