Just looking for advice on the size tin I should use, for a cake to feed 80 people. And a rough idea of how much you would charge.

What I have in mind is, I want the cake all one size so it's going to be quite tall. So I'm thinking, layer a 12" sponge cake on top of one another so it'll be 4 layers? Will that be enough?

The cake will be covered in fondant over white ganache with a little gum paste decoration on top, a 3D character of some sort. The price I'm thinking of charging is in the range of £150.

Does anyone have any advice please :( 

I can help you with serving sizes. I use the Wilton serving sizes on the web. The serving sizes may seem small but if your cakes is 4-5 inches tall they will be quite generous. The site will give you serving amounts for round and square cakes. As for what to charge, it depends on the amount of  time you will be putting into making and decorating it.

http://www.wilton.com/cakes/making-cakes/baking-wedding-cake-2-inch-pans.cfm

The serving chart Danilou posted is the industry standard in the US so we use it to figure out cake sizes for the servings needed.  For your pricing you need to figure out what all your costs are, ingredients, supplies, cakeboards, utilities, etc.  Then you need to know how long things take, getting supplies, prep, baking, icing, decorating and decide what you want for your hourly "wage".  Then there is profit and other things to consider.  What others would charge isn't really helpful because your costs and how long it would take would/could be very different from me.  Geography, skill level make a huge difference in pricing as well as what your market will bear.