Redundency Pay

I was made redundant a year ago and given a lump sum. only 20% was paid in tax so I face a tax bill of £10,000. they have now asked me to fill in a self assesement.
I am worried that the sum I was also given a lump sum which my employer said was tax free (£30k) how do I declare this without addinging furthuer to the £10k tax demand they already want?