Your MPAN (electric) and MPRN (gas) are needed by your new supplier to ensure it switches the correct supply for you.
They consist of numbers only and no letters. You should be able to find these either by looking on your bills or by contacting your current supplier who should be able to confirm these to you.
Your MPAN is your 13 digit electricity supply number and looks like this:
Your MPRN is your 6-10 digit gas supply number and will look like this:
It's worth noting that these numbers differ from your serial number which will include letters and numbers.