Anybody know what can take rust spots off sails without ruining the cloth?

Oxalic acid - get it from a decent chemist - converts the iron oxide into the oxalate and you can wash it away.  It is a minor poison but fine it handled with a bit of care - does not improve your race performance if drunk!

Thanks. Off to the chemist!

If you can't find a traditional chemist then Y10 fibreglass hull cleaner gel works but you will need more than one application on dense spots. I used it recently to remove some very bad rust spots from a set of Nat 12 dacron sails I bought to use as cruising/single handed sails for my MR.