Many thanks Karfa - you were correct

After 5 hours on call lines and 5 different agents (it didn't help me that this was during the pilot strike) I finally goto a brilliant agent who understood the problem and was able to manually adjust the taxes

for anyone else who has the problem you need to call the call centre and say the reward flight taxes sound like they are incorrectly calculated
Reward flights should be £0 . The BA system calculates the GST (K3) at full rate.
The agent needs to manually recalculate the taxes for Avios flights - I got the ticket for £208.00 not the nearly £500 they quoted originally

Also be sure to state that you tried and couldn't book on the web... you should not have to pay the call centre booking fee either