I am afraid I can't give you much advice as to what it is, but I would at the very least cut the affected leaves and stems off and dispose of them.

They look like hard-shell aphids. I agree, cut off any infested leaves quickly.

I agree with the aphid diagnosis. Cutting off the affected leaves will help contain quite the infestation, but if your plant is small, you may not want to strip it completely right after ordering/transplanting. Mint is hard to kill, but it is probably already pretty stressed. Aphids can also simply be washed off with straight water, or a very light dish soap solution. The soap helps kill eggs. You may have to rub the leaves a little to help rid the eggs off. If those don't work, I also use neem oil to organically control aphid and whitefly outbreaks on my indoor potted herbs (they love my lemon verbena and scented geraniums). I just wait a bit before using after I've sprayed them. If the plant is outdoors-ladybugs love aphids. You can try to import some onto your plant.

Thank you all for your responses :) They are so helpful, I have clipped the affected leaves & he seems quite happy, I thought it may have had something to do with the wet soil splashing up onto the leaves so I scattered a bit of barley straw around him to keep the plant leaves off the soil :P How do I make a soap solution for future reference? Thank you again

Just a little squirt (drop or two) of dish soap in a spray bottle of water.

For small plants in pots it is possible to hold the plant at an angle under the tap and gently wash the leaves with your fingers. That way you can get to both sides of the leaves. You need to keep your other hand over the top of the pot to keep the soil from falling out.