Products and services - rounding up issue.

SUGGESTED

This is happening with a couple of products of different costs. If I create a product with a gross price of say £39.99, I have to enter the net price but net of £39.99 is £33.325 and Sage rounds this up to £33.33 net, £40.00 gross. I can't get it to be £39.99 so have to manually change it in the invoice - if I remember! It's driving me mad. Any ideas?