I've played around a bit, but I can't seem to fix it. I've got a lifespan of 3.5 and the particles fade away like they should, but a few - 4 or 5 suddenly flash solid white. I've got a glow on the particle cloud - could that have anything to do with it?

No - I've mucked around with that as well - makes no difference. I could post a rendered image if that would help - if you could tell me how.

Thanks for your help