Micra K11 fuel consumption and revving at idle

My 2001 K11 998cc 68k miles only does 30 mpg in town. I cleaned the MAS with a spray and since then
it does a 5-10 sec revving to 1700rpm? at idle. Only sometimes, and not always as fast nor for the same number of seconds.
My usual garage said they would connect their computer and do something, but now they say I have to send
the "carb" somewhere to be "fixed" instead. Can somebody suggest?
Or a garage in NW London who know the ropes?

Go to a proper garage. A k11 doesn’t have a carb. The coilpack k11’s use odb2 so every proper garage should be able to see the live data and tell you what’s wrong.