Auto gearbox change down issue

[Targarama]

2010 Fiesta 1.4 Automatic. 86k miles. Runs great apart from the gearbox in one circumstance; when you slow for a speed bump, roundabout or similar obstacle where your speed is say 5-15mph the gearbox does not change down until you touch the throttle, resulting in a bit of a delay akin to the old Smart Four Two mechanical auto gearbox. It is fine otherwise, smooth changes, no bangs, no slips, no cause for concern. This happens when the car is hot or cold.

Could this be some kind of sensor? or something like the air-flow-meter? (economy and general running are fine btw). I'm probably going to get a local transmission specialist take a look if its not something obvious.