I'd go for Powdery Mildew. You could give it a blast with Fungus Fighter now or wait until the spring. If you wanted to treat is just for now, perhaps make up a weak solution of milk and water and clean the leaves with a cotton wool swab. When you plant it out, make sure to prepare the soil and give it a feed with perhaps Vitax Q4 or similar.

I reckon Jimmy is right, powdery mildew. Often a sign of dryness at the roots at some stage in its life.

Thank you very much - I’ll try the milk and water and also try to make sure it’s watered more consistently. The weather here in Warwickshire’s been a mix of heavy rains mixed with strong drying winds winds over the past months. Now we’ve got snow forecast !!