Two core rubber flex to take 500w-not so easy to locate in shops.

[hotponyshoes]

I don't think you need to connect it at the drill end if the drill is double insulated.
Ie. You can use 3 core cable and just connect the earth core at the plug end so it can't become live/Will trip the power if the cable gets damaged.

 

[Richard.]

I was just gonna say. If 3 core flex is more common and fits nicely into the gland why not just use that. There is nothing saying you have to connect all the cores so long as unconnected ones are terminated safely.

 

[Jim Davey]

The 3 core stuff is fatter making it a pita to get through the strain relief on most tools.
I fit Lineax to all the tools we repair, it’s good quality and the insulation is nice and thick which helps to protect the conductors. Don’t forget though that rubber insulation offers no help at all to stop the strain on conductors if you pull the cable. So if you like dangling 9” grinders by the lead then you’ll break the conductors sooner than if you use PVC flex which has little or no stretch.