ladybirder, we have had this convo before :) Dogs react very differently to melatonin from humans. My 30 kg dogs get 15 mg or more per dose and can have this every 4 to 6 hours when things are bad. It doesn’t even make them sleepy let alone knock them out.
The starter dose for a 20 kg dog is 6mg 3 times daily but the dose can be doubled with no ill effect and given more often. I, and others have noticed that stress seems to clear the melatonin from the dog’s system faster so it may be better to stick to the 6 mg but give it more often. Errrrr you won’t be likely to get much info from your vet as they are not allowed to prescribe or sell it. Its not illegal to own it or use it on the UK, just not legal to sell it or for a vet to prescribe it. Ask by all means.
Here’s a useful article
http://webcanine.com/2007/melatonin-for-stress/