I have had a number of people enquire about this, and so far I don’t have the perfect solution. Often the rust mark has damaged the fabric, so even if you can remove the mark, you may be left with a small hole.
One option is to mix a paste of salt and lemon juice; apply to the stain and work into the fibres then hang in the sun to dry. Once dry brush off the residue, rinse and wash. This does work quite well.
I have also just ordered a bottle of HG Hagesan Stain Away No.7 which is a rust remover product. When it arrives I’ll give it a try and let you know the outcome.