Aha I've managed to figure out whats wrong

The problem is the way that SagePay's "Basket" info has to be formatted when its sent to them. An example can be seen here on page 41. As SagePay do not allow a VAT/ tax field to be added after the shipping field then it is impossable to send this info to them

PTFE pipe cutter 1 15 3.00 18.00 18.00
as you showed here the fields sent to SagePay are:

Item description = PTFE pipe cutter
Quantity = 1
Price = 15
Tax/VAT = 3.00 (20% of £15.00 = £3.00)
Price including tax/VAT = 18.00
Line total = 18.00 (linetotal is price + VAT x quantity)
then shipping is added lastly at £15.19

I will contact SagePay immediatly but unless they change the way they allow the "basket" field to be sent to them then there is unfortunatly nothing I can do.

My apoligies, I hope this helps
Nixak