You'd be able to pick up a sigma 70-200mm F2.8 for about 650 quid and can also get a 1.4 converter to give that extra bit of range (equivalent to 100-280mm F4) on brighter days for about 200 quid.
As with everything you get what you pay for but i'm shooting with the equivalent sigma for Nokia and getting ok results.
The resale values are quite good too when your ready for the next step
if you google dalephotographic they have some good deals and i can recommend them, its where I buy all my kit
hope that helps