Get yourself along to the BMF Rally at Peterborough in May and you might track down what you are looking for.
There is a Douglas and Rudge Club and a Vintage Club.
According to George McKenzie, who spent half of 1930 at Crystal Palace after Marine Gardens in Edinburgh closed, Johnson was so"home" to Australia. When he went to dump the 1930 version, it went into a corner where his motors from 1928 and 1929 were lying dumped.