TV mount problems

[i_am_fubar]

I'm not 100% sure if this is the right forum, but I figure its closest to a building problem.

My brother and myself are moving into a new house soon, and we were putting a TV mount up the other day. Its a heavy duty mount for TV's upto 52". The mounting place was the chimeny, and fixings were provided in the form of 4x 13mm diamiter, 80mm long plugs and hex head bolts.

I drilled the 4 holes straight, and pushed the plugs in. They fitted snuggly, held the mount up, started screwing the bolts in, but the top left one started to go in at an angle, then a very quiet click occured. and a hair crack apeared between the two left hand bolts.

The plaster in that area is not hollow sounding when tapped.

The wall is breeze block, not solid brick.

My question is this:
Is this likely to be just the plaster cracking, meaning the wall strength hasn't been impared.
Or is there a chance the block work behind has cracked, weakening the whole wall.
We're prepared to move the mount down some so that the top (angled) bolt isn't needed. Does anyone feel this will cause problems? Also, as we've lost the old fixings. which can we replace them with?

 

[JerryM]

the fixings being used (and needed) are heavy duty. they aren't for hanging up mirrors.

in older houses chimneys are not good spots brickwork wise to fix anything heavy duty to. when they were built the openings were much larger (higher & wider) than today. they have a brick arch at the top of the opening - above this the brickwork will be sound and ok for the tv fixings.

below this i find the infill has been done merely to suit a need ie block off the gaps and the brickwork poor. this is unlikely to be sound for the heavy duty fixings.

without taking off the plaster it's trial an error to find the sound brickwork. i have not come across an arch higher than about 5ft so above this should be ok.

i also feel the fixings are a bit ott. i think the block has most likely cracked but not a concern. suggest you get some smaller fixings ie 6 or 8mm hole size and shorter say 60mm use something like frame fixings