I am still scared silly of these blocks.

Don't be. It is an irrational phobic fear.

Any advise about fixing very heavy items ?

Use rawlbolts. I particularly like the type that you can put the cabinets on
the wall, and adjust them with those adjustable top mountings. Then, you use
the type of rawlbolt that just requires a 10mm hole through the wooden (not
hardboard) part of the cabinet, then a 10mm hole into the wall. No need to
remove or move the cabinet at any stage. You then just hammer in the
rawlbolt and give it a final twist with a screwdriver. No constant removing
the unit, marking and drilling holes, just to find it is 3mm out.

Christian.