Wratherproofing new workshop wall on existing larger base

[bagofjuice]

DIYer in need of some help. One wall of a new large lean-to shed / workshop will not overhang an existing concrete base (other 2 sides do overhang). I had a 2 course engineering brick base laid around the perimeter. The wall face in question will have a roller shutter door but the problem is the brick base around the opening is small and is easily knocked and detached - and rebuilt twice now.

I was thinking to replace those bricks with a wooden base on the concrete, maybe with CT1, but need some advise on how best to weatherproof.

There is a drain (not parallel, urgh) about 10 - 18cm in front of that wall.

So what's the best option here?
Z flashing into the drain, or move the drain so the wall overhangs, or something else?

 

[Deluks]

Replace the wall with 140mm concrete blocks, flush to the inside of the wall. Flash the front edge over the block, and lap behind your membrane.
Secure the timber frame to the floor, internally, using a heavy duty long right angle restraint strap, screwed along its length into the timber and then concrete screws into the floor.