This website (Ofcom) will show you the location of masts. However that may not help as although you may be close to mast, if the line of sight is blocked by something that interferes with the signal then you may not get much if any reception.
Not sure why SureSignal is not fixing your problem... thought that didn't need a mast nearby.
We are on our 3rd Vodafone Sure Signal box and its not working. I've been trying to get it to work for a month, now. Its driving me insane. None of us have a signal here, and its really mucking up our family communication. I've called Vodafone, been on their forums, bought new cables, checked our broadband speeds against their requirements.
All the other networks are patchy around here - I've checked their maps and spoke to others in the village. The reason why we're with Vodafone is because of the Sure Signal, but 2 of our 3 contracts are about to run out with them, so am happy to change.
What can we do? We're only 40 miles outside London ffs!!