Yeah sorry, I keep remembering why I did things a certain way. This has been a long project with me doing bits of it in the evenings, and I haven't been making notes on certain design choices....
I was thinking that if I knew the absolute maximum size that the post data could be, I could say to the module, 'Hey, I'm gonna send 10,000b' then use the return value of Serial.print to determine how much has been written, then subtract that figure from 10000 and add null characters, or spaces until I reach 10000. Inefficient, but should work.
Or I could do what I'm doing at the moment and find the length, but when I come to send the data, I rerun the code, but instead of strcat'ing it into a big long string, I output it via serial.
If you don't send the correct number of bytes you're going to write to the SIM900 module, it just returns an error.
Oh, and my variable types are poorly chosen, I should just be using char or byte for those single b entries. I hadn't got to the stage of it working before I optimised everything. I think my assuming that large amount of RAM will fix everything has been foolish.