I would be great for a client to book without making an account if they don't wish too.

It's already possible, sorry I don't remember where is this setting but you can remove the simplybook login or any account creation.

I agree. Very important to me, I need the login account possibility to manage client and the mobile App.
But sometime they don't wish to create an account particularly in older patients.
So Could you please add both
Book Now button, AND Login to Book Button

thank you