Thanks for all the help and advice folks.
Update.
My trap has now caught two mice, though in two instances the cunning critters managed to take the bait and not trigger it.
It is possible as per upthread that I have caught the same adventurer twice.
But more progress.
I have discovered that the trap I acquired for a fiver is a professional bit of kit retailing for around £60.
This:
View: https://www.youtube.com/watch?v=Qt608TESMmA
and
View: https://www.youtube.com/watch?v=0w2stGYtzNc
and have realised from those vids and othe research that I have been using it in the wrong way - you are supposed to slide one bit out of the other when deployed - this very probably increases the chances of the trigger lever being triggered and you not giving a mouse a free feed.
It also means that you give the mouse substantially more room to sit things out - in this space you are apparently supposed to supply the critter with bedding, more food to keep its spirits up, and for all I know some suitable reading matter. Will consider this and other furnishings.
Apparently the trap was designed by Oxford academics - good to know that something other than Oxford set politicos has come out of that place.
Both mice were unharmed - the second one was taken some distance away and this time I had time to look at the rather cute thing.
Thoroughly recommended - to relieve yourself of a nuisance or, in these restricted times, catch yourself a new pet.