Looking for bike storage

Hello, I live on Norwood road (13 mins from Herne Hill and about 7 from tulse hill) and am looking for somewhere safe to store my bike (Somewhere near Herne or Tulse hill). It’s a road bike used for leisurely cycling and triathlons. We don’t have any storage suitable outside the flat or inside really (3rd floor flat). All bike hangers are seemingly full in the area, the velodrome is too far for every day use (approx 22 mins walk). Happy to pay up to £25 a month for a storage space. Do let me know if you have anything suitable. Thank you.