Thanks for your question. I am Sam and I am one of the UK tax experts here on Just Answer.
You can pay up to a maximum of £40,000 into your pension, any more than a tax charge arises on the excess, however, to offset this you can carry forward unused allowances from the past 3 years #To make this easy for you HMRC have a calculator that works out for you, what amount is available for you to pay in addition to your current annual amount - which will accurately advise this for you, you just need to put in the information asked for.
Link here for this calculator http://www.hmrc.gov.uk/tools/pension-allowance/
This is the calculator you will need http://www.hmrc.gov.uk/tools/pension-allowance/standardcalculator.htm
And results are
So you have an unused carry forward figure of £166000 - more than sufficient for the contribution you wish to make
Thanks for your response.
Despite you having unused carry forward of pension relief, which can be added to the current annual allowance, you are going to be restricted to the fact that you annual income is only £20,000 so you can only contribute a maximum of 100% of your annual income - which in your case is £20,000