Are these suitable for sensitive skin?
The unscented options are the ones to choose - fragrance is the usual culprit. Bio-D and Clara both do unscented, and rinsing an extra cycle helps too.
Can I use fabric conditioner on towels?
Better not to. It coats the fibres and reduces absorbency, which is exactly what you do not want from a towel.
Are they septic tank safe?
Most of these are, and it is stated on the bottle. Bio-D and Ecover both formulate with that in mind.
Do you sell refills?
Yes - Miniml and Wilton London in particular, and Clara's five-litre. Far less plastic and better value per wash.