A brief report back on this: I just rang up the Silver BAEC line and explained the situation. Was put on hold for a while, while the advisor checked with the BAEC team, and then was passed on to them. The "BAEC" advisor confirmed the details, including the date/value/order number of the transaction with Tesco (I'd only transferred £5).

She confirmed that I would definitely be getting the bonus Avios, and that she'd raise a request for these to be added to the account within 7 working days - which would take it to Friday 28th, I think. If they hadn't shown up then, I am to call back. We'll see what happens in the next 2 weeks.

A bit of a hassle, really, but hopefully nothing more than spending 10-20 min on the phone in total.