You are asking us? All answers you will get will be to keep holding. What was your plan when you bought? Since you obviously entered Bitcoin to make profit if you are asking when to sell. Without having a clear plan and targets set then you can consider yourself lucky to have made this kind of profit.

Sell a little by little if you don't need much money.

You take the profit and you still keep some bitcoin in your possession. Until the price goes high, you rinse and repeat that strategy so every up, you get to sell bitcoin at good prices.

But it's best if you have left a portion for long term hold.

Your choice if you dont still need some money then you can opt to hold but if you do like to diversify then you can do it but be sure on sticking with top altcoins in the market and never consider those low caps.

Able to bought when it was still on 9k was impressive because not all is really having the feeling on buying on that time since everyone was still on doubt and the call or buying was just actually right on the spot.

Time to sell your btc? It depends on your needs and your upheading plans because not all would really be having the same mindset when it comes to investment.If you do saw some opportunity
in alts then selling out some portion of profits isnt really actually a bad decision to make.

I think many suggest hodl or sell it if you need funds...
you have got a profit of 60% - 70% so the questions you ask are difficult to answer correctly. I personally follow the advice of the majority who say 'hodl if you don't need or sell if you need the money'.

So are you satisfied with your current profit? If so, then go ahead, take your profit and be happy. But, are you sure you invest your profits in altcoins? You do know that altcoins are very volatile, right? Those "altcoin season" might end anytime and if you don't exit quick, you will lose most of your profit.
I would instead suggest you to keep holding your bitcoin if you want to make decent profit in a less risky process. Still looking forward to invest in altcoins? Then invest in those that are well known. I would go for ETH.

that's £31k profit(a pretty good one) in the current price of BTC

if you ask me I am ready to withdraw it but it is your money but just like what bL4nkcode said if you have a spare budget and can still afford to hodl then why not but if you are uncomfortable with hodling any further in fear of losing profit, then go ahead withdraw or you can do what you said "withdraw the half then just buy back again". in the end it is your choice you should have confidence on what you decide.

Everyone has their plans with bitcoin and making a recommendation that can satisfy you I think is not possible. Most bitcoiners are looking to buy more bitcoin and hold. I plan to hold my bitcoin for the future of my 12 and 13 year old nephews.
Although if you want to sell any amount of your bitcoin for any altcoins that offers you some ROI, it is your will.
From what I see of the big investors is that they will not sell. They are buying more bitcoin every day in order to secure the future of bitcoin and their investments.

How much profit did you get now? If you think that your profit is enough already, well, you do the sell. I'm not going to say JUST HOLD it is because I don't know your situation but it is assuming that you are in profiting now since the price is moving high.

And if you are planning to invest in altcoins, just be sure that you are choosing those coins that really have a good market performance and high liquidity. But I suggest securing your funds to BTC while the excess money is for investing altcoins. Putting our investment into 2-3 different coins is really a good idea than having all of it in just a single coin.

Wow, you must be happy with this, holding it from the very low rate of Bitcoin.
If I were you, I will sell half of it, at least, I have had my fund back to me. ANd others are the profits and bonus for me for holding along this time. Taking profits for some of our coins sometimes is important in order not to regret it. Although we also feel sure that the BTC price may be increasing again. But for me, at least, I will secure some of them and hold some for the next Bullrun.