Thanks Backpacker for your insights...
The convoy is just going to start at random times up to about 10:00, so it won't really be Formation Flying as such., The route will be the same, so ATC will see a few of us all doing the same thing that morning.
I think I could manage 8000ft for most of the route, until we meet airspace restrictions. Hopefully we will have a favourable wind, there and back..!