Don't know if mid life crisis and entry level mix but:
Something second hand with at least: double wall rear rim, 14mm rear axle, 3 or 2 piece cranks, small chainring (25 - 30 tooth), 21"+ frame (top tube) if you're any taller than 5 feet, maybe a rear brake to start with.
If the park is good and busy then it's probably worth asking some riders around there as well. I tend to keep all the useable parts I've replaced incase something brakes
Don't know if mid life crisis and entry level mix but:
Something second hand with at least: double wall rear rim, 14mm rear axle, 3 or 2 piece cranks, small chainring (25 - 30 tooth), 21"+ frame (top tube) if you're any taller than 5 feet, maybe a rear brake to start with.
Are you London bas3d?