we have just bought a galvanised steel bench for the garden, that we thought would be the perfect thing for our evening G&Ts. BUT when the evening sun comes on it it is covered in flies...why? a cheap metal black painted one (B&Q type) doesn't get covered. Thank you.

Hard to say I am afraid. It's possible that there's some factory coating on the new bench that is attracting them (or something in the paint on the old bench that is repelling them) but without being able to do some experiements this is only a working hypothesis!