thanks for joining in on my question, I didn't expect to evoke this interest level!
So ... yes I agree you shouldn't use floats - but it's not my system I'm just troubleshooting and you can't just turnaround to, say, the MD of a company and say "well you shouldn't use floats" without some backup and solid reasoning, after all from his point of view how do you explain that the same data gives different results on different servers?
Anyway I did get confirmation elsewhere, and it's to do with the cpu's. I have hazy memories of reading about floating point precision within the cpu back in the days of the p pro and early zeons - but I don't have the original article and I don't have a precise memory. I'm told ( and for all the reasons you guys pointed out ) that I'd never get consistant results from one server to another - it's suggested just cloning the database on the same server would also produce a different result - sadly i don't have the disk space to check that one.
so thanks very much for all your input.