Just a quick rounding this one off (I think) I now have a confirmed solution on the problem, as I've got a timescale for someone running a similar sized problem on a newer machine;

My 7 year old last of the Power PC's Mac Powerbook with Excel 2004; 3-4 hours.

A less than 2 year old (don't know the specific date) Lenovo laptop running Excel 2010; 2 minutes.

I'm figuring my friend had the benefits of a newer machine and the benefits of improved calculation methods within the newer version of Excel? I'm not sure if we had the same max change settings but either way if he went to 3 dp and I went to 5 it wouldn't make 3 hours+ difference. I'd be grateful for any second opinions on this but it looks conclusive (I think)?