Hmm there is a bakery nearby but its not very big. I will call them in the morning. I was really surprised that tesco idn't come up trumps. They only had small circular ones. We're near a city and so if necessary i could find a bakers there and make a trip in.
You need a cake decorating shop, rather than a bakers unless the bakers specialise in decorated cakes and are willing to sell you one from their stock.
IME none of the supermarkets do anything other than round or square 10" boards. Finding your nearest cake decorating shop is your best bet for boards - check Yell.com, I bet there's one not far from you.
Re. the cake, yes, it will keep til Saturday, Ordinary sponges keep well for about a week, certain recipes, especially chocolate laden ones will stay fresh for a fortnight or so. Once it's iced (and therefore airtight) keep it in a cardboard box NOT a sealed box or the icing will go all sticky.
Thanks for all your help ladies. I found one! Hobbycraft sell them and cake boxes too. In fact I could have spent a fortune on all sorts of cool stuff.
Now I have to attempt icing my first ever birthday cake. Cake is done and cooled. I'm off to post under food to see if anyone can give me advice on icing it! Please do follow me across if you have top tips since I'm very nervous about messing it all up!