A few days ago, I posted
No one responded.
But, being determined, I asked the Natural History Museum discussion group on plants.
It is Puccinia Urticaria (nice pictures, including mine!)
Quote – Puccinia urticata is a rust fungus which develops on the stems and leaves of Common (Stinging) Nettle or Small (Annual) Nettle. It often starts as small orange blisters or swellings. Close examination may reveal the pinhead-like fruiting bodies of the fungus.
Anywhere that the host plant grows
The spores may spread to other nettles or to various sedges where the telial stage of the life cycle continues without causing a gall to form.
Common and widespread in Britain. End quote
I am sure your life will not be transformed by this information. Mr Furlong doesn’t seem at all excited about it. But somehow I feel quite chuffed!