Can any of you master growers out there offer a bit of advice on this please?

Quite simply, this is burning from too much nitrogen. Even though you only gave one mild feed, it indicates that your soil mix is plenty rich enough in nutrients to not require any extra feeding as yet. No need to do anything, just don't give any fertilizers containing nitrogen or high amounts of potassium. The super dark green leaves indicate plenty of N/P/K is available still. A seaweed extract or fulvic/humic acid will boost health and growth without adding nitrogen. This is something you could give them if you feel you need to do "something". Otherwise just keep giving plain water and maybe try another mild feed in 10-14 days time......quarter strength to half strength maximum. Once leaves are damaged, they generally stay damaged. Remember too......... It is very, very easy to remedy a slightly underfed plant, but very, very difficult to rescue a burned and damaged overfed plant!