I'm guessing you don't have any meters? Ok, so you wait for the seedling to be about two weeks old before starting feeding. Then fill up a gallon jug of good water and start with about one teaspoon of nute per gal. Sorry, you're a Brit so you have to convert it. You can do that. You are British after all. You guys are pretty smart as Europeans go. So anyway mix that up and let it sit for a few hours. Then give the plant a drink. A "Gift" as it's called. Watch the plant carefully for the next few days. Actually until it is harvested but pay particular attention to it at this point. if You see any shriveling of leave or anything that doesn't look right, stop giving it nutes and start giving it plain water. Weaken the solution by half and start again. If nothing but good happened you can add a little more next batch. Keep doing that same thing until harvest and you'll be fine.