would it just be a matter of putting in a tablespoon or 2 of poppy seed into my regular recipe (WASC) lemon flavour of course to get a good poppyseed cake?

Add enough poppy seeds to get the look you want ie you want to be able to see plenty of seeds in the batter. Also I would use fresh lemon zest and also some juice rather than lemon flavouring, you get a much tastier cake.