Carpentry and Woodworking - botched bookcase built by "furniture designer"

I've paid good money to a socalled furniture designer to build me a stylish floating bookshelf system. It looked great but after 18 months it's collapsed. The bookcase consists of 6 floating shelves, each shelf supported by two Twinslot 370mm heavy-duty brackets, (a British system, don't know if it's international) which are fixed by means of upright spurs to a floor-to-ceiling MDF board which, in turn, is fixed to an alcove wall. Each bracket, according to the Spur Shelves website has a max loading capacity of 100kg. I do store heavy coffee-table style books on them but this is what I told the designer guy I wanted the bookcase for and he assure me it would be ok.

When the shelves collapsed, scattering books everywhere and damaging the sofa underneath, the upright spurs came out of the wall but the MDF board stayed put against the wall. I discovered that the screws fixing the brackets to the MDF board were only 25mm long!! That means they only went into the MDF board which is 25mm thick and not the wall!!! Even I, a total novice in DIY, can see that this sytem was bound to fail. Can anyone tell me please how long the screws should be for a job of this nature?

Also I have noticed that the 'designer' adapted the brackets to fit the depth of the shelf by cutting off a strip of metal along the angled (bottom) length of the bracket to a length of approx 8cm ending at the web. Could this adaptation have contributed to the crash and if so, is there any way anyone can suggest one can get around this problem?

The furniture designer guy offered to respray the shelves and re-fix them with longer screws. He didn't say anything about the brackets and I've only just discovered that he's tampered with them. I've lost total faith in him but am stuck with him as he doesn't seem to have insurance and I don't fancy legal action. I just want him to make sure the shelves are safe but I feel I have to tell him what to do to be sure he does it.

Can anyone help please?? I'd be very grateful.

MDF does not hold screws well in a load-bearing situation. Plywood would be a better choice with the proper size and length screws.

thanks for that, Wire Puller.

MDF board is basically glue and saw dust. It's very brittle and inexpensive. I always use 3/4" plywood for the case and solid wood for the face frame and doors. Any long term vibration will case MD board to fall apart were the screws/ nails are. If you happen to drop a shelf and it hits on one of it's corners there's a 90% chance it will break as I have learned from past experience. As for the screw length, that will depend on the thickness of the wood, etc. but i always make sure the screw bites into something solid by at least a 1/2" depending on the load the shelf or whatever will have.