ShyTorque wrote: "With regard to choice of which agency to call on the radio, don't forget that GA pilots have to make that very same choice on every flight in Class G!"
I think this inadvertently misses the point. The non-glider pilots are looking for some kind of service from whoever they call, so have some idea who might give them that service. This helps them make the choice.
If I call an agency, I'm looking for no service from them - I'm merely giving them information (glider entering cloud, position and height) so that they can improve their service to other aircraft.
This arose from someone (maybe SkyTorque) suggesting that a glider pilot should make such a call, and my request for information on who I should call.
If I call e.g. Lakenheath, because they're close to me, but another IMC pilot is speaking to Essex, I gather that my call will be useless to him/her because it won't be passed on. Or vice versa.
I'd be happy to make one, possibly two calls to ATC before entering cloud, but my priority has to be 130.4 because, as ChrisN points out, the main risk is other gliders. If there's no single point of contact where I can alert other GA, it seems rather pointless for me to call at all.
One thing which does come out of this thread is that any GA pilot who wants to be sure that there is no glider traffic in cloud should listen to 130.4 for a few minutes. Gliders in cloud call frequently as their height changes, so nothing heard means (or should mean) no gliders.
If anyone can suggest an effective way to alert other GA traffic to my cloud flying I'd be delighted to use it.