I am using a form at http://www.newlifechurches.co.uk/booking/ to collect credit card details. Is there a way of ensuring the form is secure for people to use as they will be entering their credit card details?
Securing the form and your WordPress installation are not related to Gravity Forms. You need to purchase and install an SSL certificate , then configure your site to be accessed only via https/SSL. Here is one plugin which will help with that: http://wordpress.org/extend/plugins/wordpress-https/
That plugin could be used after you install your SSL certificate.
Those two things can be used to secure the information which is transmitted between the visitors and your form.
Am I correct in saying that a Wufoo form is secure when it is collecting this sort of data?
I don't believe that is a feature of their free pln, but I believe SSL is included in all their paid plans. That is a question you can ask them, to be sure. http://www.wufoo.com/signup/
UPDATE: sorry, only the $29.95/month plan and up include SSL. Their least expensive paid plan, $14.95/month does NOT include SSL