Thank you all for your advice! (And some helpful advice from Morton for once!)
It wasn't actually French horn that I played before - it was cornet and tenor horn, but it would be fairly easy to adapt I assume.
I would really quite like to play in an Orchestra - my schools is the only orchestra I would want to play in until I was of a good level obviously, then perhaps would join a local amateur orchestra. I am also very interested in conducting, composing, arranging too - I would love to be a musical director of an orchestra one day, so starting my own amateur orchestra is something I would consider in a few years perhaps. I have a great deal of friends who are also very interested in music and performing/composing too.
(I am even president of a Chopin society at school too... we actually managed to get 9 members

)
I think I will probably choose cello - it has such a beautiful sound and size doesn't bother me - I do play a piano afterall :-)
I would just love so much to one day be able to go out on stage, with an orchestra, or as part, and play something magnificent to an audience!
I think first and foremost I would love to be a concert pianist - I love the romantic concertos of Chopin, Grieg and Rachmaninoff so much! They are so emotionally evoking and beautiful, words don't even describe...
To be able to play something like that would be absolutely incredible.
Back to my original point anyway - I am going to go for the cello I think. I will be at a large music shop on Monday where I can try out various instruments (I am rather looking forward to having a go on their 90 year old Bosendorfer!!!

) so I will probably try a few there. I think it will be cello though - it has such a beautiful sound.
Thank you for all your words of advice.
Regards,
Wurlz