Hi - as a result of a rusty screw in the shower ironmongery I've rust trail stains in the grouting. Does anyone know what's best for removing it as it is pretty unsightly.

If it's actual rust, you'll need an acid or a strong chelant to get it out. I'd hate to advise you to use muriatic acid (hydrochloric acid) unless you have lots and lots of ventillation. You could try a weaker acid (vinegar, for example) and see if that'll get it to come out.

I'm not aware of any commercially-available chelants, though there may be some. Possibly someone knows a brand name.

Try Whink found in the laundry soap aisle. Whink Rust Stain Remover If it does not come off after a minute of soaking then soak some felt or cloth strips with Whink and tape over the grout so it can soak longer without evaporating. I removed a rust stain on concrete from a table leg, it took 15 minutes of soaking but worked.