Well no-one has actually answered my original question, namely a method of doing this that means that is sufficiently cheaper so that two new butts for £51 to no longer seem a good deal. As I said in my reply to Ian H, 12 blocks (of the type you suggested) plus a connecting kit is c. £30.
If a stack of blocks is the cheapest method and that will cost c. £30 then someone could have just said that and left it to me to decide how much I value my time at.
And maybe I was wrong to worry about a stack of blocks (with a 200kg weight on top) being stable and maybe I was wrong to think they needed protection from the elements. In which case I would have thought saying so, maybe with reasons, was more helpful than rolling eyes at my stupidity. People don't post questions in the areas in which they are expert but in the areas where they need help.
BTW, the solutions you personally have suggested may be straightforward to implement but I don't understand how they would work, so in that sense they are not straightforward solution.
I was originally thinking of placing the blocks with the 440x215 side down, two running left to right, then two running front to back then, two running left to right; to give a height of 300mm, the minimum to get a watering can underneath.
Earlier you said "6 concrete blocks will do the job safely, or 4 and a piece of wood across the top" and I don't see how I can get a height of at least 300mm from four blocks and a piece of wood, unless you are thinking of having one of the layers with the 215x100 side down.
In this post you said "you only need six concrete blocks laid on their side in an wide H pattern". Again the only way I can think of getting a height of at least 300mm is with one of the layers with the 215x100 side down.
Without some form of connection between the blocks, both of these sound very unstable to me.