OK - Have just installed shelving - so here's how I did it for the record - as this method seemed to work well:
Many bits are put together from kind folks' suggestions here, some from my own research. I'm not taking credit for all of it - rather just presenting a coherent method that works for me in my little cheese house.
Problem: House is made of Thermashite/aircrete/somesuch blocks behind plasterboard dot n dabbed on. Blockwork isn't so much friable but rather some loose powder which someone gobbed in and vaguely shaped into cuboids. Even by aircrete's standards, these things are *poor*.
1) Get 8x80mm (or bigger if required, bigger is better) Fischer FUR frame fixings (or similar). These have nice teeth that expand outwards over 3/4 length. Some frame fixings only have a few serrations right near the end and in our application the screw does get that far due to thickness of thing being fixed (or get a longer screw).
Worth the money and the special order in this case. I really do not think many other plugs (certainly none that B&Q, local hardware store or Homebase had) would have stood a chance, or they would be hanging on the PB mostly. I wanted good fixings into the main part of the wall as these shelves will have heavy and expensive things on.
2) As instructed, drill hole with indicated size drill bit - being very careful not to wobble the drill. Straight in and out or the hole will be oversized. Do not use a masonary bit, use an HSS bit or the hole will be oversized.
3) Blow out hole - I used a cheap "can of air" inserting the pipe right into the back of the hole.
4) Mix some rapid set (15 minute) Araldite. Generous blob round the end of the fixing and a bit along the sides and a generous smear around the bit that will be level with the plasterboard.
5) Insert plug(s) to full depth. Remove screw if used to get purchase on plug. Wipe off excess glue.
6) Smear oil on screw - cotton wool stick works well. Only need a light smear.
7) Do up screw - power driver or drill works best with this Fischer part - but if using a hand driver, keep pressure on as the supplied screw has a very shallow thread and can strip the plug whilst it is "fighting" the plug by expanding it. Finish by hand - with the oil on, the screw comes to a very definate stop when done - over tightening a lot may strip the thread cut in the plastic. In fact, even with the thread stripped, these things still hold, but mental confidence drops...
8) Over next 10 minutes back of each screw a bit and re tighten - just to break the bond of any glue that got on a bit of the screw that the oil didn't. In practise I had no problems...
Job done. I don't think those shelves are going to fall off any time soon.
The purpose of the glue to to stop the plug turning if the screw is removed (my shelves are coming straight off again for room painting, then back up). If the plug turns a bit it tends to lose its grip in the block as the powder disintegrates further.
I did try the first 4 screws dry as intended by Fischer (none of the glue/oil malarky). 3 actually held quite well but the 4th gave way and I was able to remove the plug with my fingers! Probably a combination of a slightly misdrilled hole and over tightening. That's when I decided to take the glue tip on board...
Cheers
Tim
PS - Bollocks to this in the future when I hang a telly on 4 screws close together with a significant turning moment - I am now definitely going to use resin anchor sleeves and bolts.