When I refer to my concerns about using blocks I am not focusing on
the advantages in terms of insulation but about the possible drawbacks
of mixing bricks (from my neighbour) with blocks (my property), i.e.
is it structurally OK?


New concrete blocks tend to shrink slighly whereas new clay bricks
(ie. recently out of a kiln, so not the case here) tend to expand
slightly. The effect is only really of significance over a storey
height or more. Your bigger problem would be if lime mortar has been
used in the original construction as there would be different
structural responses to movement of the walls. In this case I can't
see that there would be significant structural problems but I wouldn't
be surprised if a non-structural crack forms at the interface. You may
get reflective cracking through to the surface of the plaster unless
you take steps (movement joint in plaster/building paper+EML etc).