On return to idling

[assynt road]

Not me, the S1. On returning to idling the engine revs drop back to 300-400 revs before recovering to tick over at 900. This sometimes stalls the engine when its hot, now SWMBO is giving me grief about it, so has anyone got any idea whats causing it? The engines a 2.8, by the way so has mechanical injection only, I think! thanks, Steve.