Well we got back safely after an extra week, and with no charges from BA. Thought I’d update this thread with what I learnt in case it is of use to others.

• the policy was called ‘mid travel illness’ and ‘illness mid travel’ to me by different agents.
• everyone I spoke to was aware of the policy but seldom the details, suggests it is rarely used.
• there is a policy document the agent has access to - my top tip is to get them to talk you through it and keep notes, we had an extra hoop to jump through because the original agent didn’t explain the full details.
• we called up a few hours before our return flight and the reservation was instantly suspended - there doesn’t appear to be any benefit of ringing up days before, may be best to sit back and see how things pan out.
• BA require a doctors note stating that you are unfit to fly and an anticipated date of when you will be able to fly. This second point wasn’t initially explained to me and caused us to have to get a 2nd doctors note.
• You email the note to the provided address, what they didn’t tell me was they operate a 48 hour turn around to process, during this time your flight disappears from MMB and you are unable to rebook. In the end they took a little over 48 hours to process my email.
• Our UK based online doctor (DrCareAnywhere) was a godsend, same day video appointments and immediately sent letters which were accepted by BA.
• Once the 2nd letter was accepted BA offered to rebook a flight that evening, in the end we took one the day after. Just required any availability not Avios.
• Rebooking was under the same reference number.
  

The whole doctors letters was a bit of a pain, particularly as our travel insurance company didn’t require any evidence, they just accepted the claim.

Hope this helps others.