Thank you for your question.
The Helios 5 cleanburn needs a minimum of 100mm all around the sides and rear for optimum performance. Here is the info from the user manual on the "Downloads" section (link below):
"The stove can be recessed into a suitably sized fireplace built from noncombustible materials, but a permanent free air gap of at least 100mm should be left around the sides and top to obtain maximum heat output and for access to the rear of the stove."
The Helios is NOT suitable for a 12mm hearth and so should be placed on a full constructional hearth of 125mm thick. Full details on hearths can be found in Building Regulations Part J (link below).
The distance from the front of the stove to the front edge of the hearth is set in Building Regs Part J to be at least 300mm (Unless otherwise stated by the manufacturer).
I have 300mm in front of my own stove and wish I'd gone deeper as I occasionally have ash falling off the door onto the carpet.
Helios 5 User Manual: https://www.directstoves.com/index.php/downloads/dl/file/name/helios-5-instruction-manual-oct-16.pdf/
Building Regulations Part J: https://assets.publishing.service.gov.uk/government/uploads/system/uploads/attachment_data/file/468872/ADJ_LOCKED.pdf
Hope this helps?