It should tell you on the label how long it needs to be in situ before rain falls...

Have read and read instructions three times but no mention of watering or rainfall after application.

It needs to have dried on the leaves before the rain, otherwise,yes it will have washed off. Wait a few days, if it worked then any Dandelions will raise their leaves and they turn a sort of browny/purple. If that does not happen then you will have to begin again.
Been there, done that!

No, you don't need to do it again - I was so irritated at the lack of information on the label regarding immediate rainfall I rang Scotts, the manufacturers. They've said lots gets taken up by the roots, particularly if it rains, and advise you should wait at least four weeks to see what happens - if there are some weeds still growing, then just spot treat those after 4-6 weeks.

Thanks Bamboo and Owdboggy. I will keep an eye on the dandelions. I will just have to wait.... Had rain again all day here in Shropshire so not very hopeful.....

Well, keep your fingers crossed and bear in mind this particular product is only meant to be applied once in any year - its pretty strong and may affect the grass if you don't wait to see if its worked. Its one of those hormonal weedkillers that first causes speedier growth which should be twisted and distorted before it kills.

Maybe only a use once stuff, but no good if the weeds do not die. Supposed to kill Black Medic and Prunella, but it don't.

Here the buttercups respond before the dandelions.My weedkiller instructions say apply when no rain expected for 4 hours. Just keep your fingers crossed for now!