I tend to err on the side of caution... In my boot, I have:
Spare wheel, small socket set, screwdrivers, adjustable wrench, wheel brace, multimeter, tyre pump, wd40, carb cleaner, big torch, jump leads, gloves...
The list goes on - and I've actually used all of that stuff in a breakdown/non-start at one time or another...
Also carry my green flag membership card, as when you blow a driveshaft thorough over enthusiastic gearchanging, no amount of screwdrivers are going to get you home!
On an aside - I know a guy who used to carry a spare engine on the back seat of his Hillman Imp. It came in useful once, when the knackered engine in the back gave up the ghost, so he stopped in a layby, and spent a few hours in the sun changing the engines over by himself. Quality breakdown kit there!