My aussie when free running at all speeds in the garden or over the fields moves correctly with lovely reach and drive but as soon as you put a lead on she paces any ideas.

Increase the speed that you are moving her at.

You're probably moving her too slowly; try setting off quicker so that she trots.

Turn her in a circle before you set off, and yes increase the speed you run at. Have had this problem!!

"Turn her in a circle before you set off"

You will see professional handlers in the States/Canada automatically doing this.   I always thought it was basically to 'collect' your exhibit before setting off.   It was quite difficult to do with the long Basset, without tripping over him

Try running her on your opposite side when training. It worked for me.

I have tried running faster, starting of fast, turning in a circle towards me and away and round me with no effect in fact when I ran fast she just paced faster. Just don't know what else to do.

Give her a little 'nudge' sideways with your leg to knock her off balance when you start to run

Could you teach a hand target, and get her to elevate her head as she moves. Some obedience trainers will also try to break this habit by tucking a ball or toy in their armpit. If you intend breed showing, this might produce an incorrect outline for breed purists, but once the habit of pacing has been broken she may begin to trot normally.
Personally, I have never had a dog that paced on an incline. I would try trotting uphill, and a reachy walk downhill.

Try putting a hair bobble on a diagonal pair of legs. Hopefully with enough training and work you can train the muscle memory to adapt.

I did some searching to find 'how to' and this is one place I found.

https://petcentral.chewy.com/dog-physiology-how-dogs-move/

Scroll down.

In my breed, it's usually to do with some constructional problem I'm afraid.