Thanks for all your responses. I can't post the whole code here, of course, but I'm going to try and produce a minimal program. Hopefully some of you wouldn't mind taking a quick look at it for me.

As for the Keil/SDCC thing - the original code was written under an external contract so we got the source and binaries but not the tools. I wanted to make a simple two line change which would not affect the customer but make it a bit quicker at the manufacturing stage. So I really can't justify buying Keil just for that, and I thought SDCC would be a quick way to do it.

If I can get it working with SDCC then I know that, if we need to do more modifications in a few years time, we will be able to freely download the tools and run them on whatever version of Windows/Linux/Mac is current at the time. If I bought Keil I can't be sure it will even work on future versions of Windows. On the other hand, if this credit crunch gets any worse, in a few years time we all may find ourselves sitting in a cave and bashing rocks together for a living...