Is cellar/basement used as a living space?

A living space is a room where you might spend a significant amount of time, e.g. snug, playroom, study etc.

Is it vented to outside?

Vented means it has windows, air bricks, specialist vents etc.

Increase ventilation by clearing vents, adding vents, or adding a fan to blow air in
OR
install a sump with a fan

Install a sump with a fan
OR
install a positive ventilation system

Other things to consider:

• Seal around doors and other access points to the main house, with a draught excluder, rubber seal etc., especially if you are adding a positive ventilation system
• Ventilate any other underfloor voids
• You may need a specialist contractor to advise on the position of the sump