One member of the Allotments UK forum, going by the name of organic has informed me that I appear to have mildew on my leaves, here is the post in full.
don't mean to be disheartening but your pumpkin leaves appear to have powdery mildrew
Products containing the following chemicals are effective against this :
the organic way would mean taking all infected leaves of but it doesn't look like you have any that aren't. so probably would kill it
Ensure plants have adequate water, especially those in containers.
Keep roots moist by incorporating organic matter in autumn and applying mulches in spring.
Avoid overcrowding plants or thin out crowded growth, to give good air circulation and minimise humidity.
Don't give susceptible plants too much nitrogen-rich fertiliser because this encourages sappy growth which is more susceptible to fungi.
What organic didn't realise, that after harvesting The Accidental Pumpkin, more stems have since grown from the plant and more fruit and leaves are appearing, so I have been able to prune away the affected leaves.
Thanks organic for the advice.