I agree with cozie, wait until the weather warms up a bit before you actually make them poo outside. Meanwhile gradually start moving the litter tray nearer to the door to the garden. Do it slowly otherwise they will wonder where it's gone and just use the vacant space.
Once you've got the tray to the door you can begin to put it outside and then move it towards the muddy bit.
You can then put some used litter on the mud. take them over to it and scratch their paws in it to show them what they need to do.
Sounds a bit of a mission, but it doesn't take that long. I know some cats will just use outside without a fuss, but you don't want the risk of finding yours will go anywhere inside if the tray is just removed.
As cozie says, make sure the mud has been nicely dug, they do prefer it specially prepared.
PS - out of interest, do you have lots of cats around your neighbourhood? The second boy (he who actually started pooing inside) did this when we moved to a new neighbourhood where there were lots of rough and mean stray toms. He could hold his own if he absolutely had to but I suspect he didn't like having to - so came inside to do smelly business to avoid leaving too much scent outside.
I can't hold out too much hope AlliterationAlley given that I've had two cats who went outside but preferred to poo inside, and who can blame them - nice and warm, clean, safe etc.
One of those two actually started his life with us by using outside but started to poo inside about 4 months into his stay, even!
I wouldn't take the tray away ay the moment, personally, in case they kept on pooing inside but started to use the linen cupboard or behind the sitting room sofa or something. What you could do is dig over the flower beds as soon as the weather gets even a little better. (It's snowing with me so I'm going nowhere near the outside right now.) The earth outside at the moment is hard, wet, cold and packed/crusted - so very difficult to dig properly if you only have soft paws to do it. It would be bad enough with a fork!
Once the weather improves and, hopefully, they have nice fluffy dry-ish earth to dig in you might see an improvement. I'm assuming from your post that they pee outside so it sounds to me like a digging issue and not that they don't know about going there.