TTL was going to out of the question unless I wanted to pay top Leica prices and the two available flash guns are a bit larger than I really wanted. I checked out other manual/auto flash units but none really took my fancy.
I decided to search on Ebay for a Leica CF. I've tried a friends Leica CF, originally designed for the 35mm film Minilux, on my M8, and measured the trigger voltage at a very digital-camera-safe 6V, the search resulted in a freshly listed Leica CF at a very good price from a seller with excellent feedback. I pressed the Buy It Now button.
Two days later it arrived. It works perfectly and will live in the Billingham for those rare occasions when a little bit of extra light is needed.