I cant believe that you all seem to have differing ideas as to what the answer to a standard procedure is!!!!
firstly if you are on thames radar then overflying any field in there control area would almost certainly mean entering the london tma!! as it has a base of 2500ft for most of their coverage.Switching one box to another without informing the radar controller could lead to dire consequences!Dont do that!!
and i would have thought the correct trx initially would be "thames radar, request change to stapleford on 122.25" and in my experience the controller would probably have asked if you were intending to call them in plenty of time anyway!as he can see where your present track will lead you in a few minutes time!! What puzzles me though is why you would need to ask this question if you are the holder of a ppl and if you have forgotten in the passage of time,then why not go for a check ride with your instructor and log the hour you need for the once a year rule???
Safe and pleasant flying to all!