Ok so I now have £700 invested in bitcoin .
If that amount isn't a lot for you, my advice would be to not take your profit off the table anytime soon--unless you have good reason to, i.e., you need it for something else. If that's the case, however, you probably shouldn't be gambling with money you don't want to lose.
I've always been a buy-and-hold investor and I don't focus on short-term trades, so I don't get too excited when one of my investments goes up in value a lot in a short period of time. I think it's best to just let it ride and see if it makes you even more money. But if you are interested in getting in and out of your positions quickly and taking your profit, it's really hard for anyone else to say when you should do it. You could set a % value for when you're going to cash out, and that's probably the best way to do it, like when your bitcoin investment rises 10% it's time to sell. That's really up to you, though.
You never know what bitcoin is going to do. You could end up selling and missing out on the rocket ship to the moon.