What type of paving is it? If concrete the soil might be very acid. How large is the back garden? Does it slope down? In any case dig down to get a " core sample " so to speak to find out what type of top soil you have...sandy to loam to clay or any combination thereof. Then what type of substrate do you have..sand, broken shale, solid shale...this you will have to know in order to determine your drainage situation. Get a soil test kit to see what kind of amendments you will require if any. If everything looks good then you will not have to do much working into the soil, if not, you will have quite a job laying in what you need to grow.

Umm...more likely alkaline than acid, Loosestrife. That slowly corrects itself in moist climates, and the process can be speeded up by adding organic matter. More drastic measures, such as sulfur or copperas are rarely needed in the UK.

For some reason I was thinking of muriatic acid when I wrote that, stand corrected thank you.

Thanx for your help. I'm really looking forward to starting this project and will probably be asking more questions before I'm finished.

Had you thought of advertising the slabs on Freecycle? There is nearly always somebody wanting free ones...
Difficult to know what you'll need to do until you know what's underneath. Keep us posted!

Good luck with it and mind you don't hurt your back!
Please take some photos before,during and after so we can admire your hard work.