I have been in contact with the Potato Council and had advice. I removed and bagged all the tops down to ground level, then 3 days later lifted the entire crop and left to air dry before putting into hessian sacks. We ended up with around 50Kg of quite small but fairly nice looking potatoes of various varieties. I reckon this would have been more like 200Kg if they gad fully matured. I am now monitoring these the check for signs of rot in storage.
The Blight is apparently largely down to the climate we have had over the past few weeks. Frequent rain so the leaves rarely dry out combined with humid and warmish conditions. The perfect combination for Late Blight.
I have enrolled as a monitor for for the Potato Council Blight Watch Scheme and have submitted samples of the foliage and stems for lab analysis. They keep interactive online maps of where attacks are occurring: http://www.potato.org.uk/node/910
The service provides SMS alerts when there is an elevated chance of blight in members area and enables them to take preventative measures including spraying with Copper Sulphate based fungicide. This is only effective if applied before attack and is not a cure unfortunately.
Information can be had from the Councils Website: http://www.potato.org.uk/gardenblight
A few pictures of the plots after clearance and the crop below: