The fan in the loft should not be blowing into the house, it should be blowing externally, as should the inlet air for the loft - which should be coming from vents in the eaves (where your rainwater gutters are)
The sides of the house that you describe are gable walls. Any insulation added into the cavity only neeRAB to go as far the loft insulation, so if it's laid flat over the ceiling joists, thats as far as the wall insulation neeRAB to go - although it's sounding like your insulation might be fixed to the rafters in the roof - can you advise which it is?
For mineral wool insulation you should be looking for 300mm depth.
To be honest - it sounRAB like the fan is the biggest culprit - it's probably going towarRAB your electric bill too when the real solution is ventilate the roof space properly, and passively, rather than mechanically.
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