For one you dont use wall plugs with coach bolts lolol Also you wouldnt use an 8mm fixing with a 10mm plug , you'd use an 8mm plug with an 8mm fixing!
Sorry , i'm not trying to be a smart a**e but so many people use this site for advice and the advice here is excellent but if morgan was to follow your advice his beloved screen would fall straight off the wall!
Morgan , sleeve anchor fixings are the way to go , i'd use at least 8 and make sure theyre about 100mm long to ensure a super strong fixing! Theyre easy to use and the instructions are on the back of the pack! Theyre also cheap to buy at around £7 ish for 4 , you can pick them up at your local d.i.y store or goto
www.screwfixdirect.co.uk and type 'sleeve' or 'anchor' into the search bar and it'll bring up a whole range of fixings in different sizes and designs!
Also it may be worth scanning back through a few pages in this area of avforums of the home diy section as this subject has been covered a lot in quite some detail
Hope this helps!
Jon