Hi
I have an LMP mount for my HS10 and was lucky enough to find a joist in the way for 1 of the 4 mounting points. I contacted the rawlplug technical advisory service
http://www.artex-rawlplug.co.uk/immediacy-43 as I was considering using their cavity fixings into the other 3 holes which were/are just in plain plasterboard ceiling. Brian was very helpful and told me the weight one of these would bear if suspending something from the ceiling - it's different weightbearing if you screw it into a wall, apparently.
http://www.rawlplug.co.uk/Products/l...y.htm#Interset
You can get them from B&Q, screwfix, diytools, etc
I ended up using the spring toggles as they have a bigger "footprint" and I could get easy access to the "inside" of the ceiling - the attic is above the ceiling so I can get to both sides of the plasterboard - but I recall Brian saying that the "interset" fixings actually bear more load.
Anyway, give 'em a call. I can't remember which one I used now or what weight he told me but I do remember that the weight 1 would bear was essentially the weight of my projector and since I was using 3 of them (and a joist) I didn't worry. Mount is up and in use and has been for several months now and is still solid as a rock.
Good luck!