Dave... really sorry to hear that!
Apart from the obvious ice and rest, you need to get a good ankle support asap and wear it while you're walking around during the day. It will help your ankle to heal quicker while you get on with day to day stuff.
You don't need to worry about training... you've got loads in the bank and your fitness will not noticeably deteriorate.
If you take complete rest from running for a week, wear the ankle support, and take it easy, you could probably gently test your ankle in a weeks time. Don't be tempted to try and play catch up though if it's ok.
If it still needs more rest then take another 3 or 4 days. I'm almost positive it will be fine by then... and you'll be ok for the half
