Thank you thank you.

(I love this forum!

)
I used a 340g marmalade jar about 1/3 full. Very very thick (remember we are talking Jersey here) and it really did take less than 10 mins shaking in all. 5 mins to go into thick cream that you thought you couldn't shake any more and probably less than 20 more shakes for it to clear the jar and be butter.
I refrigerated the previous night's milk, skimmed the cream off (by hand with a spoon) in the morning and left it in the jar in the cool larder, left the morning's milk standing in the larder, skimmed it when the cream had risen and added it to the jar, then shook it straight away.
My thinking now is that everyone talks about washing it and working it to make it last longer, but maybe the less work way is to just make each day what you want for the next 24 hours - the 'off' flavour won't have time to develop, if yesterday's experience is anything to go by!
I suppose I should make extra, properly washed and worked, to freeze for when there's less creamy milk available, and/or when the ambient temperature is too cold or too hot to make butter.