can i train an adult cat to go to the toilet outside?

[Idrankthechristmasspirits]

She still refuses to go anywhere other than inside but won't go in the litter tray, she poos next to it. I think she is probably too big for a litter tray, we have the biggest one available.
She is (we think) about 2 yrs old. (rescue cat)

It's driving me potty though, any advice will be fantastic!

You could try putting down newspaper under the litter tray so she goes on that, then move the whole lot gradually nearer the catflap, then finally outside the catflap - that might get the idea through to her in easy stages.