I’m not sure which thermostat you mean.

    Is it one of these http://www.cleghorn.co.uk/c-warm-heaters/c-warm-heater-accessories/cw268-thermostatic-blender-valve-15mm.htm?

    If so, it should be fairly straightforward to replace.

    If you haven’t got one of these fitted (they were not fitted as standard on earlier boats), then it is well worthwhile fitting one. Not only does it avoid scalding hot water from the taps, it also makes the hot water go further by mixing the (scalding) hot water direct from the calorifier with cold water before circulation to the taps.