Thank you both for your replies. That's encouraging to hear.
Although this flat hasn't had serious money on it, and the kitchen and bathroom could do with replacing, the location within the neighbourhood is the best.
I've noticed that the expensively- renovated rentals with the estate agents are usually in much worse locations, being far from the park closer to the main road. So perhaps this flat's micro-location may make it equally appealing to renters despite the rough edges.
And although it's not in prime zone 2 (Clapham, Brixton, Balham, Hackney), it's a second-tier neighbourhood that many like.
Yesterday I heard from an international student offering to pay 6 or 12 months upfront. I replied instantly saying look, I've has scores of replies including from professionals with large salaries, but if you'll do that it's yours, please come for a viewing any time (they were in London).
Four hours later, feeling peeved that he still hadn't opened my reply, I had great pleasure auto-rejecting him!
For now I think I will slightly increase the price on the OpenRent listing and see how that affects responses. As I haven't scheduled any viewings yet I don't feel bad about doing so....