Take leaves and fruit to a local nursery.

Or at least tell us where you live!

I live in England, South West

Looks like a quince to me.

Me too. Chaenomeles speciosa

the leaves look like Camelia to me, but i've never seen the fruits of Camelia...

Looks like quince to me as well. We have a large tree in our yard that I used to think was an apple tree. Now I'm thinking its a quince! It is about 15 feet high. Last year it produced a good amount of fruit that was eaten by the deer, crows and squirrels. They left me a single fruit that was as large as a grapefruit. It wasn't quite as sweet as an apple but it was still tasty.

Here are two links that may answer some of your other questions.

Links that might be useful:

http://deliciousdays.blog.com/2012/04/09/learn-about-the-characteristics-and-uses-of-quince-fruit/

http://www.gardeningblog.net/how-to-grow/quinces/

Thanks for your help. Just moved house, we have had to get rid of a few poisonous plants already.

Just looked at camellia fruits, it looks like it could be that! I will see how it develops. Thanks again for all the help and advice

Camellia japonica. It's evergreen, right?

They are Camellia fruit. Even without the fruit there's no question.

Pic is from a park near Sacramento, CA

Yep, Camellia japonica pods. They will turn brown as they ripen, and split open at those seams you see. If wanting to grow camellias from seed, they must ripen and dehisce on the plant; this will happen in the fall. Camellia breeders will bag the capsules while ripening in order to ensure the prompt collection of the seed. They're fussy.

I've put seeds in a baggy with moist sheet sphagnum and put them in the fridge until spring. When I pulled them out, every single one had germinated in the moss and was ready to plant! So much fun.