This cache is NOT at the listed location - although free parking is.
To obtain co ordinates for this cache you need to complete the Rocla Art Trail Adventure Lab cache series.
You need to use the Adventure Lab App on a phone and visit the 5 stages answering questions. Each correct answer at each stage gives you a 'find' and will increase the number of lab caches you have found each time.
Once the trail is completed, in the app, select "View Journal Entries" and scroll to the final (5th) entry to receive the final co-ordinates for this puzzle.
Good luck, and I hope you enjoy the first Adventure Lab in Milton Keynes!
Here's the link to the adventure lab; NB: You'll have to have the Adventure Lab app installed to complete it:
https://labs.geocaching.com/goto/RoclaArt
P.S. If you enjoyed this art trail, then you might enjoy the original, running alongside the canal in Great Linford: https://coord.info/GC6P1Y1