Thanks everybody for your help. The problem, as Andy points out, is that you can find quite easily the 15A plug, that is much larger than the one I need. Clarification on the BS-546 plugs can be found at
BS 546 - Wikipedia, the free encyclopedia .
The 2A plug is the smaller of the three and it is the difficult one to find.
Dear Greg, you are right, as always: the instruction manual of the 409 states (pag.61) that "...the trickle charger socket....is to be used for an inspection lamp or for a trickle charger..."
I'll go on exploring the links that you kindly provided me.
Have a nice day,
Stefano