OK - reporting back. Outbound BNE-DXB-CDG, Expert Flyer showed middle seats on both segments blocked before check in. This was confirmed at check in.

Returning CDG-DXB-BNE, Expert Flyer (when I checked) showed a vacant seat on the first leg and blocked on the second. At check in, the agent blocked the middle seat for the first segment - in this case it then showed as occupied (not blocked) on EF, the agent said she couldn't do anything for the second leg, but seems it was already blocked anyway.

So in summary 3 seats between the 2 of us on all segments, so not too bad. We certainly noticed the narrower seat on the B777 compared to the A380 - I would have hated to be 3 abreast for 14 hours on the B777.