We are new to fish keeping and have a new (1 month) 12 liter setup.

Filter in use came with the Pets at Home set.

Since setting up we have conducted 2 x 20% water changes adding a drop of Fluval water conditioner each time.

There are 3 Platys in the tank; 2 females and a male.

The issue is only currently present on the male

Would somebody be kind enough to help me identify the ailment please?

Looks like some sort of external parasitic infection which will need treated ASAP (along with ensuring that water quality is at optimum levels at all times which is crucial to reduce the risk of any medical problems). Check weblink here: [to follow]

For water quality, use a liquid-based test kit such as API Masterkit (not strip-based tests) and ensure that ammonia and nitrite are consistently at 0 ppm and nitrates no higher than 20 ppm above your tapwater nitrates level.

PS. I tried to edit my post above but the edits vanished.

Treatments such as eSHa's gdex or alx, Waterlife's Sterazin or Parazin, or Interpet's Anti-Crustacean Parasite are likely to be best. Check their descriptions and internet images of what they describe and see what seems to be the best fit.

Thank you. I'll go do some Googling based on your suggestions.