15 min will always be "free". Podpoint's way of working is that you plug in and go. After you get settled down, you "claim" your charge.
I've been doing something similar at lunchtime with Leaf back when my workplace charging isn't so busy. Plug in for 15min, have lunch, if need more charge, plug in for another 15min, go for a walk, and then return to move the car. They used to unlock the cable at charger end so a quick unplug and plug back in will reset 15min. But now it remains locked until you unplug at car end.
Unfortunately now, unchanged at 12p/kWh, it seems most EV owners are hovering around the charger wanting to get a cheap charge. Work is reluctant rising the price, fear people might have committed to salary sacrifice EV budgeting on this benefit. Despite (with my input) the charger policy states along lines "do not rely on using workplace charger, home charging should be primary source "
Luckily with my longer range EV doing commute, I don't need to bother charging anywhere else.