Thanks for the quick, accurate replay. I asked BA.com the same question via webform and here was the reply:

Thank you for writing to us about your upcoming travel plans.

We try to offer as many reward seats as possible on British Airways flights but there will only be a limited number available. This means that on popular routes at high-demand travel times the reward seats get booked very quickly. If you are planning to travel at peak times, we do recommend you book as early as possible to increase the chance of booking your first choice of flight/date.

As flight departure dates approach we also consider allocating additional seats where we have sold less tickets than anticipated. If you need to make a booking at short notice, it’s always worth checking first if you can use your Avios points.

I hope this information is useful to you.

Kind regards

TO me this doesn't address what I asked, but hey-ho.

Thanks all!