To Find This Cache you need to complete the set of labcaches (National Waterways Museum). On correctly answering the question at each location you will need to answer a further question to help find this bonus cache.
The labcaches are in public areas that can be visited at any time without having to enter the museum, though a visit is very educational and enjoyable. The bonus cache is then a short walk away.
On completion of the five labcache stages you should have collected the values for A through J. This bonus cache can then be found at :
N 53 17. (I) (G-B) (F+J) W 002 53. (D-A) (E) (H-C)
Please take care with children and pets when doing the labcaches and the bonus cache as (unsurprisingly) there is deep water around the National Waterways Museum.
To access the labcaches, scan this QR code (or click on it) from your smart-phone,
Or click on this link : https://adventurelab.page.link/wKX1