Well, I took the plunge and left SFR to join Free to take advantage of this new deal.
Everything Free has said will happen has happened AND when they said it would! One small annoyance - and totally my fault! - I'd forgotten that my mobile was provided through an SFR contract and obviously doesn't work with a Free SIM card. Went out today and bought a new mobile and, once the Free SIM card was inserted, it worked immediately.
Let's see what happens when it comes to invoicing and customer back up if needed. But so far so good!