Smouty, you are absolutely right. Working on another installation with an ASUS RT-N56U wireless router (an amazing piece of kit) I came across some notes that if you use a WPA/WPA2 mixture then wireless will drop back to 802.11g. So I changed my BT Hub from WPA/WPA2 to WPA2 only and bang!! Now I get 36mb down!! Problem solved.
I can understand why it's set this way to allow for older devices, so I guess that's the price we pay for rapidly changing technology.
Which reminds me of when Sir John Harvey-Jones (in the Troubleshooter TV programme) said to a fellow in the Morgan car factory chassis shop - "You must have seen a lot of changes in the 35 years you've been here..' To which the man looked at him blankly and said 'No' !!!
If only IT was like that... as if! Anyway, thanks for comments - maybe fixing this problem will help someone else.