Ta dah! My new raised bed is finished...
My hands are sore, but the rest of me is happy. It looks just how I wanted it to look - big, sturdy and deep. The inside dimensions are the same as the old beds, that's to say 1 metre x 2.4 metres. With the 10cm thick sleepers though, the external dimensions are 20cm bigger in each direction.
But the biggest difference is the height, which at 40cm is more than double that of its predecessor.
By the way, the bed is positioned over open soil too, so theoretically plant roots can go down as far as they want. However, the soil down below is very poor - sandy and stony, and containing lots of builders' rubble from when the house was built.
The bed is surrounded by shingle now, since the paving-stones are all going to be removed before long.
The wooden sleepers are quite heavy (33.5kgs per 2.4m length), so they would almost stay in place just with their own weight and the pressure of the soil within, but I have held the corners together with steel angle-brackets, secured with some long screws. I would have liked longer brackets, but unless I wanted to pay silly amounts of money, these were the biggest I could get.
To hold the two layers together I used straight brackets like these:
Should you be interested in constructing a bed like this for yourself, here is a costing for you:
6 x 2.4m sleepers @ £21.65 = £123.24 + £18 delivery charge [Travis Perkins]
8 x 6" angle-brackets @ £1.35 = £10.80
2 x 6" straight brackets @ £1.00 = £2.00
56 x 3" screws, approx £3.00
Total cost: £157.04
Add to this the cost of shingle of course. I paid £71.64 for a tonne of it (including delivery), but I have only used a small fraction of it so far, and there will be enough for all the rest of my planned alterations too. [Jewson]
Now I know you will all say "I could have got it cheaper", but I take into account time and convenience as well as materials, and I am well satisfied with what I got. To buy the Woodblocx bed like the one I have (which is a very similar size) would be £321.98.