You will be paying more than you might think.
I am leaving the company within the next couple of months, and given the volatile state of the share price, I picked a decent time to cash out, expecting to pay a 1/3 of the total back in Tax + NI, so 33%. How wrong I was. It turns out, you pay 40% total back which is significantly more than expected. Fortunately, I am not reliant on the cash, but if you are making plans for the money, it is worth considering, as it is more than the quoted 1/3 being discussed here.
I phoned up payroll to check, who said it was correct and also phoned HMRC to confirm, and she said likewise, so it's one to suck up, but it is a significant amount to lose to the taxman!