Building a step

I need to build a concrete step from my back door down into the yard as it's currently a drop of 250mm which my disabled partner and ageing dog both struggle with. I'm looking at a size of around 900 x 450 x 125 mm which I work out to be 0.05 cubic meters and to be honest I'm getting myself totally confused with various DIY shops offerings of various concrete, cement and ballast offerings. I'm also on an incredibly tight budget (every penny really does count), and since I'm far happier dealing with electricals and electronics than sand and cement I'm after a little help.

Using Instant Mix Concrete 25kg Maxibags I figure I'll need 5 bags, but if I use cement and all-in ballast I figure I need 1 bag of cement and 6 bags of ballast which is something like 2/3rds of the price, but I'm not convinced I've done my sums properly........

It would be easier and cheaper for you to use a couple 7N dense concrete blocks. They come in 100mm*215mm*440mm - so put a couple laid flat buttered up and bedded down together and ta da! You've got yourself a 900mm*215mm*100mm step. At 80p each I would use 4 and plonk a 900mm*600mm*50mm flag on top to finish it off. Lovely jubbley

I did consider using blocks, but my skill at buttering and bedding blocks is pretty much zero, and I'm likely to end up with something far more dangerous than the exisiting drop.

I'm far more confident that I can build a wooden mold and shovel concrete into it without major problems. I'm just concerned about getting the right ratio of cement to ballast, and roughly how much of each I'm going to need.

Yes, you're sums are right..
A bag of cement and 5 bags of ballast will do your job.

Don't forget to check the DPC level.

The DPC runs right underneath the concrete plinth the door frame is resting on, just course of mortar separating them.