I agree with Matt about not wanting any step to stand on - if the lip is behind you you may well step back off it and lose balance. I have my lathe on an mdf board and it doesn't extend past the end of the lathe legs. It still gets under my feet sometimes when I want to be really near to the work. And it sometimes gets in the way when I want to slide things from the floor to under the lathe, like my extractor.
18mm concrete will crack horribly, especially around where the lathe legs sit, unless you put a lot of reinforcement into it. Go with the ply solution. Epoxy mortar under each leg might be a possibility, but again, I'd wonder about cracking.
Idle curiosity... why put something under all the legs and then level with Gripfill? Why not get is on a level and then use slightly wedge shaped supports under the legs that are left floating?
How about putting it on some conveyor belt type material.
You could bolt through this, so fixing it to the floor, it would dampen vibration, raise the lathe & also prevent damp rising.
Bloody hell Tom, I've got no better suggestions than others given, but I thought your other lathe was big enough to do telegraph poles. A bigger one :shock: