I need to get rid of mice in the loft.

The problem with mice in any loft / attic space is that they tend to burrow themselves under the insulation. The dense insulating material acts as soundproofing which means they can't hear the ultrasonic sound. The insulation is preventing the heat from escaping. Unfortunately it is also the perfect environment for a mouse to inhabit. 

In summary, an ultrasonic repeller won't really have much effect in a well insulated loft. Buy some traps or poison.