Yes, you need a ticket, please see the topic in either the Probefest forum, or the store forum.
Full member tickets £10 per Ford Probe or main car
Non full member tickets £15 per Ford Probe or main car
Which means it would cost you £15, or if you became a full member for £10, you can buy a ticket for £10, meaning for £5 more than a non member ticket, you get a full years membership.