I have a fibreglass shower base (white) that has dark stains that resist exit mould and everyday shower cleaners. What would you recommend please.

Ive had good results on some really bad fiberglass shower floors using The Works T-bowl cleaner and a scrub brush. Squirt some on the floor add water and scrub with a long handled kitchen brush. Then rinse good.
Make sure you test a spot. Ive never had a problem but you never know. Also don't leave it on too long before you rinse. And be sure to vent. The fumes will get you. PHEW.

Sometimes the floor finish is so worn that the undercoat is showing. In that case forget it.

Home Depot has recoating kit for fiberglass showers and tubs. Ive never tried it but you might want to check it out. Good luck, Tom

I have used a thick layer of borax or ZUD on a damp surface. You may get most of the stain out but it will likely stain again if the surface is etched or shot. Tom is talking about something like Rust=-o-leums Tub and Tile kit. Works great but the fumes will knock you off your feet.

Bob is right. Have used the tub & tile kit. It will give u a light head. Must be used with good ventilation and exhaust fan. I also used a hepa filter mask.

Chuck