Oh you don't need to buy it! Your help has paid for it many times over.
Firstly, for the other ones, i wasn't setting up he SRAM info correctly in the header. Which is odd, cos I had it working fine in a previous game, so I assumed I hadn't changed anything.. but somewhere along the line I'd deleted it!
But for all the others, I remembered vaguely that you have to pad the game out to the cartridge size, so I wrote a little tool to do that and patch up the header. I also fix the checksum in there, but I don't think that's used by anything. At least Kega Fusion doesn't complain about an incorrect checksum now though! :)