Posted: 26/04/2013 at 00:22
Hi garjobo, while I don't really fit the description very well , I am pretty handy at DIY and will try and help. First, breeze blocks come in 3 types, low density (sometimes called aerated), medium density and high density (sometimes called concrete blocks.) You want medium or high density. Medium density can be drilled fairly easily and are strong enough to take rawlplugs. High density will be much harder to drill and are much heavier. Don't use low density - those are the crumbly ones.
First, lay your base out on the concrete and make sure it is square by measuring between opposite corners. When both measurements are exactly the same, it is square (ie all corners are 90 degrees.) Lay the base on the concrete slab (which you say you have already laid) and, using chalk, draw all the way around the inside of the base. Using a straight piece of timber, mark another line about an inch inside the lines you drew on the concrete before, so you end up with two rectangles, one inside the other. Lay the breeze blocks to the inner lines using a 3:1 mix of builders sand and cement, so there will be one inch of breeze block showing all the way around the inside, when you lay the base on them. This gives you some leeway and will ensure that the rawlplug holes (which you will drill later) aren't too near the edge of the blocks. Wait a few days to make sure the cement has set (cover with plastic if rain or frost is expected.)
Drill 5mm holes through the bottom of the metal base (about 4 or 5 holes along each side will be enough, and 2 or 3 at each end) then lay the base on the breeze blocks and mark the breeze blocks below through the holes (eg using a permanent marker pen.) Remove the base and drill holes for the rawlplugs at the marks, using the correct size drill bit to suit the rawlplugs (I suggest 6mm rawlplugs.) The holes need to be 45mm deep (wrap some electrical tape around your masonry drill bit, so there is 45mm of bit showing from the tip, so you don't drill the holes too deep.) Screw the base into the rawlplugs using size 5x45mm stainless steels woodscrews (eg 5x45mm turbo ultra from Screwfix.) It is important to use stainless steel screws when fixing aluminium anywhere which will get wet as even zinc plated steel screws will rust quickly. If any of the holes are in the wrong place, you can still unscrew the base at this stage and drill new holes.
Hope that helps!