Welcome to the forum Claire :)
Unfortunately I don't have any kids (yet), been actively trying since a miscarriage at 12 weeks in August. Right now i'm taking folic acid supplements, and my partner is taking Zinc and Vitamin C supplements to help his little swimmers along! I suppose you could try those if you aren't/haven't already. I haven't heard of the cough medicine idea, wondering how long it'll be til I give that a go hehe.
Judging how quicky you fell with your last 2, I can't see why you'd have a long wait so good luck and keep us updated!
xx