As people still seem to be reading this thread here is an update. After eight bouts of the scotomas through the summer (they were continuous through June - August) they disappeared at the end of September. We live in southern England and the National Pollen and Aerobiology Research Unit at the University of Worcester kindly sent me pollen counts for near this part of the country. The scotomas started with the onset of the birch pollen season but it was not obvious that the gaps coincided with changes in pollen count. I did a DST home FastCheck POC IgE blood test for inhalant antigens about a week after the last scotoma and there was a weak positive to birch pollen and stinging nettle pollen.
After doing the IgE test I was sent September nettle pollen data for Worcester (which included all members of the Urticaceae). Levels decreased to low about the 11th and almost disappeared by the end of the month. The scotomas could therefore be a reaction to pollen but I will perhaps try to do an IgE test for tomato allergy next year if the scotomas return.