If it's free why do I have to input my credit card details?

You'll still need to enter your card details to finalise the transaction, even though the will is free. There won't be any charge made to your card.

Octopus Legacy do offer a £10 a year subscription service that allows you to make unlimited changes to your will for free for the first year and only £10 per year after that. 

You can cancel this service at any time. 

They offer this because a will is a living document so most people update it every 5 years or every time they have a major life change. Traditionally this was really expensive - involving starting from scratch each time, or paying hefty fees for will amendments (codicils). 

Their will subscription service allows you to easily amend your will at any point AND save money over time (even if you only update it every 8 years).