I use concrete breeze blocks 9" x 18". Bury them 3" on edge and have a bed 6" high , or bury them 6" on end and have a bed 12" high. Lay them flat and have you beds 4", 8", 12", etc. Mine are now 12" high and I am looking forward to them being 16" when I can sit on the wall to do my gardening. Cost of blocks in my area vary from 52p to 69p. Catalogues show 8' x 4' x6" raised beds and sell at about £40. 36 blocks will give you the same area by 8" high at half the price. I use the top of the wall to grow some veg in pots. I'm still experimenting with that. Has been good with root crops and peas but don't expect them to be of show standard.

Bigal