Thanks again for commenting everyone.
Cheers, Mike. For the rain I used Vallejo's Water Effects. It's a gel that dries perfectly clear. If you let it get a little tacky you can shape and pluck it, like I did for some of the smaller drops on the bunker top. I used it for the drops in the trees too, using a toothpick to place them on. In hindsight I should have made them a little smaller.
For the taller drops on the bunker and puddle I used stretched clear sprue with a dollop of Water Effects on the end. The ringlets in the puddle were just painted in dilluted white oil paint.
Cheers mate, have a good weekend!