In reply to JJL:
> Does "2/5" mean "bet £2 and get £7 back if you're right" or "bet £5 and get £7 back if you're right"?
> If it's the former, then the bookies seem to think we'll be staying in - which is interesting as the markets seem to fear otherwise.
Your reasoning is back-to-front, but so's your guess, so your conclusion is correct!
It's the latter, which means you have to gamble more to win less, showing that they think you are likely to win.
The bookies seem moderately confident that there will be a remain vote somewhere between 50% and 60%; this hasn't changed much over the last few months.
They're generally a better guide than opinion polls, as I understand it, but it's still a gamble.