Ok Foodie, if £1000 includes scaffolding, then that sounds about right. They'll have to prepare, and put on a scratch coat on the first day, then come back for the second coat, but they should be able to scat it (roughcast) on the second day as well.
Be prepared for more of the render to fall off when they start poking around. The price should not rise by very much because once you're up there, it's easy to put on a bit more.
One thing I would do .............. roughcast over the WHOLE wall including the old render. New scat is very sharp. It will show through the paint. Scatting the whole wall will bring it all to one reasonably consistent texture.