I get so sick of fixing others' mistakes...

Hi - I've posted a couple of times, before, mostly about issues the previous moronic owners have caused (I wanted to move two hanging brackets in garage the other day, sure enough, screwed into drywall only, not a piece of wood underneath either - basically, I can't trust anything they've done!)

They had a shed built - it's reasonably professional so I know they didn't do it ;) - the first problem is obvious, there's not enough eaves overhang, rain hits the doors etc badly.....walls are routed ply panel - seems to be delaminating on a couple of edges, not a good sign but it's not undercoated/painted yet....and the doors are only chipboard, with the wet weather they're not looking good.

Anyway, I've got water coming under the wall in a couple of places, and it's staining the ply cladding - obviously it's not going to last long at this rate...

I can help things by extending the eaves/barge boards out front and back, and taking some of the spare asphalt roofing and wedging it in (not sure how I can add from the edge without it leaking though) so I've got some overhang, but in the meantime I need to stop this water running under.

The dirt at the back, where it's worst, was built up quite a bit, so I assumed that water was running down the hill and straight onto the concrete slab (unfortunately the slab is completely flat so the bottom of the wall plates rest level with the flat of the slab outside, there is damp proof course, but water is running right under or over it, obviously the slab should not have had any overhang, or it should have been lower or sloping outside.)

So I am considering my options - I thought mastic or some sort of silicone at the bottom edge of the wall, this won't protect anything per se but it should stop the water at the edge of the wall - but I'm not sure how it will stand up to repeated water exposure, obviously I need to paint it (with a decent primer, maybe oil-based would offer better protection?) as soon as possible, but is silicone my only option? It won't trap water in, any that does get through will seep through to the inside of the shed....

I had considered some aluminum angle, set up against the edge of the wall, but this obviously won't help if the water's running from above, but what if it was fixed directly to the wall and well sealed, above and down to the concrete, or would that be delaying the inevitable?

Would I be better prying the bottom of the cladding up, and slipping some aluminum sheet flashing under it? if it went up under the cladding, then down and across the concrete, then downwards over the edge of the concrete, would that have some chance of success?

Any ideas would be appreciated, I'm kind of thinking out loud at the moment.