Thanks for the responses. I checked the level this morning and seems to be full or near full. So no urgent need. I will call the Cub dealer (here in Toowoomba, Qld) this morning and get him to get some genuine Cub Cadet Drive System Fluid Plus fluid for me to have here (I'm 40 mins north of there).
Obviously after the mow yesterday the fluid (and there's 5 qts or4.7 litres of it) was still in the hydraulic system and has now drained back to the sum overnight.
Cheers, Peter