Right. Apparantly, my shipping needs to work like this.

The vast majority of products all have a £5 shipping charge(Lets call it item A), and even if you buy 6, or how ever many, the shipping stays at £5. Some other products have a £11 charge (say item B) which is charged per item. So if you buy 3 of item B at £11 postage is will cost £33. If you buy 3 of B and 1 of A, it would be £33 + £5 = £38. If you were to buy 3 of B and 2 of A, it would still be £33 + £5.

If that made any sense to anyone and there is a way to make this work, I'd be very grateful to know how. Although I'm not holding out much hope!