Assuming you got 1/4 odds for a place (it might even be 1/3 odds), if she comes second you would get £1250 profit plus your £50 stake back. On that basis, given that she looks like she's heading that way I would be tempted to leave it in place unless the £600 would make a huge difference to you.
In other words, if you are a regular gambler and have lost ££hundreds before and can shrug it off if you lose, then let it ride. However if this was something of a one-off bet, you're a bit skint and you could really do with the money for Xmas pressies, car insurance etc etc etc, consider taking the £600 just in case she comes 3rd and you get nothing back*. It's guaranteed profit after all. That said, your cash-out figure should be more like £1000 at this stage... unless there's not enough people wanting to back her.
Of course, if Saara makes it to the last week your cash-out position could be even stronger, and if she wins you'd be in for a bumper return of over £6000. She certainly seems to have the momentum at the moment (you could have got 22/1 only last week but now it's just 3/1), however it's difficult to work out her true popularity because a lot of foreigners are skewing things. FWIW, I have moved more of my money onto Matt than Saara recently, but then I haven't done Each Way bets like you have.
*Just one caveat: you seem to think the bookies will pay-out on three places, but e.w. bets on this market are often two places (Winner and Runner-up), so just double-check that before you make a decision.