Hi SpottyGreenFrog
Kind of hard to design a garden without actual photos and scale drawings. I spent two years designing my own garden and learned a lot about the process. Maybe this will help:
Draw a scale plan of your existing garden including all features of the house (doors, windows, water tap, drain pipes), the boundaries (if they are fence panels or brick with pillars like mine etc) and any existing plants you're keeping.
Then pick a prominent feature (say patio doors, your shed, a feature you plan on having in the garden) and create a grid based on its size.
Grids are used by architects, town planners and professional garden designers. It's a neat trick of the trade that helps them organise the space, control the position of elements, meet aestethic requirements (it helps to visually create a harmonious link between your house and garden for instance) and believe it or not they make planning and designing your garden easier.
The way you do this is you draw lines from either side of the patio doors to the back of your garden, then parallel lines spaced out by the width of your chosen feature. Then at 90 degrees to that you draw another set of parallel lines. These are spaced either by the same width if you want a square grid or another measurement if you have a second feature it would make sense to echo in your grid.
For instance if you chose your shed, you would construct your grid by drawing a straight line from the corner of the shed first towards the house, then from the same corner towards the side with the block of flats. Then you draw lines parallel to your first lines spaced apart by your shed dimensions.
It's a bit of a faff, to be honest. But once I cottoned onto the grid thing, I had a lot more success coming up with designs that worked in our space and it helped me see what wouldn't work. If you're not computer savvy and do this on a CAD program like Google Sketchup, you can always draw out a few grids and then photocopy them so you don't have to do this over and over again (this is what I did once I had settled on the two grids that made sense).
You probably need to try a few different sizes - in the end I drew all my designs on square 1.5m and 2m grids, even tried a few diagonal ones.
Those you create (taking your shed as an example again) by turning your ruler to a 45 degree angle and then drawing a line from the corner of your shed to the opposite corner of the house, then parallel lines to that based on the shed dimensions, then turn your ruler by 90 degrees again and draw from the corner of the shed to both garden walls etc. So the grid is again a whole load of squares or rectangles, just kittycornered.
If you're interested in more, I can upload my original drawings to let you see how this looked for my garden.