[DrNickBurton]

"I thought I had found a superb deal with Jessops offering the Sigma 100-300 F4 at £599 and had intended to get a 1.4x converter. The reviews on this lense are brilliant but sadly Jessops now say that the proper price on this lense was £1,119."

I noticed that price too - it's a shame it wasn't for real. I guess it was just too good to be true...

Anyway, to the OP:

If you budget is £1000, I think you can choose between a new lens and a new camera, but probably not both. If you go for a new body, and you decide to stick with Nikon, it's a straight choice between the D5000 and the D90. They both have the same excellent sensor inside, the differences between them are predominantly ergonomic. I'd recomend you go and try handling both in the shop, and decide whether or not the D90 is worth the extra money to you.

Lenswise, as others have mentioned, the Sigma zooms are very popular. The Sigma 50-500mm and the Sigma 150-500mm are both available for less than £1k. Either of these lenses will work fine on either of the bodies mentioned above. They are really heavy monsters though, and not the sort of thing you want to carry around all day (not without a monopod at least).