Should there be a warning when the screen wash is getting low, just managed to empty the tank without a warning.

Don't think so - the same thing happened to me the other day. I was expecting a symbol on the dash too.

It is on mine which has headlight washers and consequently the larger tank, however it may not be on those with the standard tank.

I have the warning light and beep. Elegance spec' 4x4 with headlight washers.

My 4x4 has a warning beeb and light for low washer fluid.