Thanks Smiffy.

Why angle iron? I was planning to attach a square bit of timber to the outside of the top of the shuttering, and just cast the rebate for the boards straight into the concrete. When the concrete sets and the shuttering (and the bit of square section timber) comes out, there will be a suitable recess for the boards.

How do you hold the angle iron in place when casting concrete round it?

I can see angle iron would mean it would be easier to slide the boards about, but is it needed for strength?

Angle Iron also for the repeated movement of the boards - concrete on corners and edges will just erode and brake off over time, where the metal will act a little like a lintel over a door - it will take the wear and tear.

Maybe worth looking at the jacks from a 4 post ramp, set the angle at the right width and you could run one of those jacks on it.

the angle gives the edge strength cement it in place or pin it and then pour the slab make sure its sealed/ dammed up or the pour will head off down your pit!

lift! as all the stuff on the car that you think you need to do from the convenience of the pit is accessed from the side of the car. A pit lets you look at the blank underside of the chassis. A lift lets you work on everything at a back friendly height.

I dug a pit, finished size 6ft long and 3ft wide and it is great for the 2cv. The depth was my shoulder height. I cover it with3' x 6" x 2" planks. just use a step ladder to get in and out.
As it's not that long I work on the front or back and with the 2cv it's easy to move it from underneath.
Only thing I didn't do is line it and under extreme conditions it used to get water in it but hasn't done so lately.
Don