Thanks Helly, that's it - are my Assos shorts too tight!!. Ah no, pressure (new to me saddle and new to me position prolly) on the nerve.
Definitely not sciatica (not painful, just odd feeling!), nor a repeat of the pain I had on the other side, which apparently came from a nerve trapped around about the T4 vertebar, but resulted in a sharp pain over the (right) hip bone. A Chiropractor I went to at the time said I was too flexible.