I've got a lower corner of my house and the corner of a nearby garage
suffering from dissolving mortar. Because they are separate buildings I
can't work out what it could be - rain water blowing into the
brick/poor guttering or a mains leak underneath the house...it's a
newish house still under 10yr house builders guarantee - just- perhaps
it's just crap mortar!

Any ideas on what would be the best way forward? Is this a job for a
surveyer?