I bought one of them 'pony' 25 quid ones.
It's ok if you know you have a flat battery and want to start. but if you are thinking of turning the engine over a few times to diagnose a fault, forget it.
Wish I had laid out the cash for a good 'un
Costco sell one can't remember the make think its RING, the battery in it is a 20ah, it also has a tyre inflator and cigarette lighter socket, light etc, don't know if its any good or not. Sur eits about £50