I'd be inclined to vote with my feet - if you've only just started the policy this week you can cancel it with a full refund. Whether or not they are obliged to honour the original 'advert' consider whether this is a company you feel comfortable dealing with.
I'm not sure I agree that it is a minor thing and I believe you should write, you should phone and you should complain if you think you have been misled BUT make sure you've got Alfie fully covered for ALL the things you feel are important first. Go to another insurer if necessary. Remember that most insurers will not cover anything that arises within the first 14 days of cover so get him fully covered first then deal with Petplan.
Good luck
Thanks everyone, i have contacted trading standards and they say as petplan have a 14 day money back offer, then they dont have to honnor anything they say on their website ( unbelievably! )
Unfortunatley this is something i may have to swallow, having looked around at many insurance plans i still find petplan are the best cover as regards to vets bills ( which is what is most important to me ) so i think id be cutting my nose off to spite my face if i went elsewhere. I just think its very unfair to advertise something that they dont provide. The policy booklet states it is an "insurance contract" and as such, im expected to adhere to it, its just ashame petplan dont have to. I am very careful about reading my documents when they come through, but many others may not, and may assume wrongfully that they are covered when infact they arent.
A lesson learnt to me is do check the certificate properly!!!