By 'maturing' I am assuming the money has been in a cash ISA for a given time frame, eg 1 or 2 years?
If you want to invest it instead of just leaving it in a low interest account, you could transfer it into a Stocks and shares ISA (keeping ISA status) and drip feed it into one or more funds, plus add more ISA money into them if you have spare cash, as this years allowance.
Or put say half into another cash ISA, half into an instant access account, and use the latter to drip feed into a S&S ISA, but this will then count as this year's ISA allowance.
There are a number of fund supermarkets you can use, most have a range of general funds, with varying risks, that you can pick. Pick more than one, drip feed money in monthly and you should gain over time, more than any interest paying cash ISA.
The recommendation is to invest for 5yrs minm but you can access at any time, and if the market does drop, by doing a monthly payment you will smooth out the peaks and troughs of the stock market. And if you do want to cash some out, you could choose to sell from funds that have increased the most, leaving others to grow more.