Finally I got decided there were no mice left and could get rid.
So I carefully gathered up the little trays, wrapped them in several layers of protective plastic bags and read the instructions:
DO NOT THROW IN THE NORMAL RUBBISH. Dispose of by burning or burying
Hmmm... I'm kind of garden free here - and sneaking up to a nearby bonfire over the recent weekend seemed a poor idea. So I called Greenwich Council:
Me: Hi - I have some rat poison to get rid of.
GC: Why don't you throw it in the rubbish.
Me: because it says not to do that. It says to bury or burn it.
GC: So why don't you do that?
Me: because I have no garden.
GC: Ah... please hold. [music] Take it to the recycling dump.
So I drive to the dump and approach the nearest worker and explain I have some “rat poison” to dispose of. “'Rubble' goes in that bay he says.” No... not rubble, rat POISON. I show him the plastic bag containing the plastic bags containing the poison. “Oh... plastics bags in that bay.” No... not plastics bags - they're containing the RAT POISON. So he nods understanding, takes the bag from me and departs.
I'm not entirely sure I didn't see him throwing it in household waste as I drove away...