You've not stated how long you've been on JSA. If you were employed during the early part of the current tax year (i.e. from April 2009 onwards) you will have been paying PAYE tax during that period. Some of that tax might now be due back to you because your taxable income throughout the whole of the tax year is less than what the system was assuming it would be.
For example, if you were earning £400 per week, you would have been paying about £55 per week in tax because the system assumed that your taxable pay for the full year would be £20,800. If you lost your job 20 weeks into the tax year and then received only £65 per week of JSA for the rest of the year, your total taxable income for the year would be £10,080. That means that your tax liability for the year would be approximately £720. Since you'd already paid £1100 in tax, HMRC would have to repay about £380.
Chris