Hi, thanks for the suggestions all. My photos are stored on the SD card, although I recently transferred most of them to my PC anyway just in case that was causing a problem. I do delete texts periodically. Just tried deleting them all just now as an experiment but it had no effect on the free memory.
When the memory is low I always empty cache first but there is usually not much in there so I have to resort to deleting data. Interestingly, comparing with my list from a week ago, Facebook and maps have accumulated another 8mb between them. So I can sort of see where all this data is coming from but there's not much I can do about it. If the phone didn't have such a stupidly small memory I don't think it would be causing a problem.