Hibernation Substrate for a TGG

[Truss]

Having a dilemma over substrate for hibernation of my 75 years + TGG Spur Thigh tortoise. Have built a box within box setup with temp guage for the first time after inheriting my tortoise from my elderly Mum. All good to go but have some hemp bedding already but some people say use it and others say use 50/50 topsoil and playsand. Most topsoil seems to have nutrients in the UK and not sure which way to go.

Any help or advice would be amazing as hibernation time is fast approaching - thanks in advance