Take a look at HS320 here for information on how this type of gain is taxed.
If you were a 40% taxpayer in 2013/14, regardless of the addition of the 28 March 2014 gain, top-slicing relief will not apply and you will have an income tax liability of £46 (£228 x 40% - 20% basic rate tax paid).
If you are a 40% taxpayer in 2014/15, regardless of the addition of the 21 June 2014 gain, top-slicing relief will not apply and you will have an income tax liability of £806.60 (£4,033 x 40% - 20% basic rate tax paid).
If you think you have a tax liability for any tax year, you are supposed to register for self-assessment by 6 October 2014 following the end of the tax year. However, as your gain is so low and the tax liability so small, I suggest you call the tax office on the number here, tell them about the 2013/14 gain and ask whether it can be assessed to tax outside of self-assessment by the issue of a P800 tax calculation. If so, HMRC will also send you a payslip to pay the tax.
For the 2014/15 gain, they may be able to do the same again or you can register for self-assessment after the end of the tax year using a form SA1 which you can find here. An online version of SA1 can be found here.
I hope this helps but let me know if you have any further questions.