Hi Mike
By way of update, I used the card Tuesday to Thursday last week, running up a refund of £2.10 and ending with a card balance of £17.80. Just touched in at Dartford this morning and my balance showed as £39.90, so the refund plus auto top up. Would be interesting to know if auto top up would have been triggered if the refund had taken me over £20. Anyway it certainly looks like the Sunday night calculation re-queues the refund as you summised in another post.