If it's the ActionChallenge event then you do not really need to carry much. The stops are well stocked. A couple of litres of water should be plenty plus a small snack for the earlier stages. Also during the day you will be going through areas where you can buy food and drink in a real emergency. It is very hilly in parts so I would be reluctant to weigh myself down with unnecessary stuff. The only thing I found that endlessly drinking water, especially from plastic containers, becomes quite unpleasant, so a small amount of concentrated squash to add to your water container might be an idea.
I used my Amazon voucher to book an appointment but made an honest mistake due to ambigous wording on your website. You won't be able to rebook my slot at this late stage so a goodwill guesture would have been nice instead of sticking rigidly to your T&Cs and demanding I pay a further £29 - so much for your promise of mind and body relaxation. Mind you serve me right - I should have been wary of an organisation that uses 'holistic' to describe what is just a basic deep tissue massage.