Thanks for the tips. I was indeed able to upgrade the cheaper WTP fare using Avios but I had to call in to do so. The end result is that I am flying outbound in CW (so can try the new JFK service) and inbound in F for £1,317 plus 48,000 Avios which is a fare level I was happy to pay for those classes. This ticket prices up at £1,935 so a saving of £558 giving me a rate of 1.16p per Avios.