Just seen your equipment so guess u know more than me, I had no joy getting what I wanted in our neck of the woods for my A300 and tried a couple of shops when i was down London, in the end i have just settled for a standard Sony 0.5 M Cable Release.
Great speedway pictures buy the way, I used to be a 1970-1973 supporter of Wembley Lions (Ove Fundin ) & Wimbledon Dons (Ronnie Moore). My dad had a couple of rides for the Lions in the days of Bill Kitchen and my claim to fame was being kicked in the shine by Nigel Boocock at Wimbledon after he lost a race.