It would be easier to ID with a picture of the plant Begoniamatt, as the "habit" of the plant helps - its shape, texture of bark, buds etc all contribute. It could be a leaf from an ash sapling - how big is the plant this leaf is from?
Please could you edit your question & add a couple more pics?

OK, I've found the picture of the tree. As I wrote there, this is an ash tree. It's really not in the right position here. If you have a LARGE garden where you could plant it - a good way from anything destructible - you could try moving it. They do get very big and are prone to "throwing" branches unexpectedly. We had one in our last (3.5 acres of woodland) garden and it dropped major (6+ inch diameter) branches every few years. Good news if you have lots of space for them to fall in and a woodburner to dispose of the debris.

Thanks very much Beattie- Sorry if I made your analysis difficult! -How long before it might become 'large' & are you talking sort of 40-50 feet tall?

We're talking about a forest tree here Begoniamatt. Rate of growth and eventual height depend on the conditions. Look up Fraxinus excelsior, or Common Ash.
Here's wikipedia on it -
http://en.wikipedia.org/wiki/Fraxinus_excelsior

It's described as "fast growing" and you can expect a tree to reach 20-35 metres in height (65 - 115 feet). Not one to allow to grow in the average garden.

My tree supplier's website suggest a height of 5 metres at 10 years old.