1. The starter has started bubbling after about 10 mins - is this normal? The instructions just stated within 24 hours!
not normal, but nothing to worry about. Sometimes the yeast gets a bit eager and gets going before you expect it to.
Also, it may be bubbling for other reasons than fermentation: if you shook up the bottle, it may have released some dissolved gas. either way,
dont worry, relax and have a homebrew.The fastest bubbling I've got was from a culture of wYeast Flying Dog Ale yeast. Got going in about 15mins and after 30mins the krausen was gushing out the top of the bottle. At this stage I took it off my desk and put it in the shed in an icecream bucket!

. this stuff has made some really good beer and even better ... I still have a culture left in the fridge

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2. How long will the starter last - if it stops bubbling before I pitch it is it still ok to pitch or do I somehow need to "revitalise" it or will it be dead ! (I plan to pitch at about 12C and then ferment at 10C)
It won't be dead, but leaving it long enough will slow it down. Making another starter from the yeast in the old starter would be your safest bet, but you may be OK without this. I've never actually left a starter too long so can't comment much. I guess the trick is to plan your brew times so that the starter is ready when you want to brew.