My understanding is that you have to apply for your Schengen to the first country of entry and/or the one where you'll spend most time. We want to travel to France (Corsica!) from UK so that rules out going there via NL or Belgium. And, we could (say) get it from the Belgium consulate and spend a day trip in Brussels first but (a) we'll want a multiple entry visa and the Belgian site says that they'll only issue a visa if Belgium is your principal country of visit (I love the way they assume you know all your travel plans for months ahead!) and (b) that's more expensive than a visa service (c) I don't want any risk, however, remote of the French immigration cutting-up rough on arrival. So, I plan to use a visa service to get me one from the French, though I'm grateful for the suggestions.
Any more agencies that people can commend?