If the creaking stops when its raining, it will be the bushes, you may be able to check by dowsing them with water. Inside edge trye wear would normally be cause by the tracking being out, towing out to much for inside wear, this would normally affect both tyres although the passenger side is likely to wear more. The only fault that could affect one wheel only is incorrect camber angle but this is unlikely as it would affect the steering/handling and is normally caused by damage - or very worn bushes.

Ive got a berlingo with exactly the same pronblem description as your fiesta, and it was both the anti-roll bar links, one was worn out and clonking, the other was siezed and creaking

What I don't want to do is buy the wrong part, I was thinking maybe the lower suspension arm but maybe not. What I want to do is inspect and see and I've got in mind you can lever under the suspension somewhere to see if there is play but I'm not sure where to apply the lever.