Usually, a quarter of the lump sum will be tax free, and the balance subject to tax at your highest marginal tax rate.
In order to withdraw your pension pot(s) in one go, a calculation needs to be made to determine the value of your pension fund, to ensure that you are eligible to take the pension out as a lump sum. The Pensions Advisory Service has plenty of information on how to value pension rights.
If you have several pension pots, there is a 12 month limit, so all the pots should be commuted as a trivial lump sum in one year.
It is also worth calculating whether you have paid the correct amount of tax when you receive your trivial lump sum, because often the pension company will deduct tax regardless of whether it is payable or not.
If you require advice on whether you are eligible for a trivial lump sum, feel free to call on 01554 770022 and we can arrange a no obligation appointment.