Levelling an eco base for shed

[Siclark]

I ordered a 14x11 workshop to be installed in my garden. I told them I’d be installing an eco base and they said ensure it’s flat but agreed it’s ok.
I’ve used those bases before but never this size. I assumed the bearers they were using would cover any small discrepancies in the base.
Over the 4.3m long side the base is off flat by 5mm, a small high spot in the middle means the edges drop off slightly.
When they tried to install they said it wasn’t good enough and refused to install. They will come and do it when the base it perfectly flat.
Their suggestion was shuttering the base and pouring self leveling compound over it so with 5-10mm over the top of the gravel filled base to give perfectly level top.

My questions are, will this stay solid?
Should I use self levelling, I’m looking at Setcrete external from wickes , for the whole thing to fill in between the gravel or do in between the grids/gravel with cement so set the gravel as aggregate and then put self levelling over the top?

For extra info I used soil stabilisation under the grids, digging cement into the soil at 10-20% at 100mm to further stabilise the soil under the grids so hopefully the soil and grids shouldn’t be moving.

*yes I’m wishing I just did concrete base now.

 

[Siclark]

I’ve just seen that Setcrete Exterior is referred to as non structural. Guess that means I shouldn’t put a heavy building in it at point loads via the bearers.

 

[scbk]

Is there no way of compacting the middle high spot with tamper etc?

To me, 5mm is nothing on a shed really, it will settle down over time

 

[Siclark]

I don’t think I can tamp it. The grids are filled now and the soil under is so solid it won’t budge. I’d have to lift all the grids and gravel.
I didn’t think it was much either but it’s a very heavy workshop and vertically clad so when they put the 2 long wall pieces up they were pulling apart.
A lighter shed and some screws and putting the roof on I’m sure would have pulled out together but they wouldn’t risk it.

My concern on the setcrete is isn’t structural

Or if I use cement sand mix to pour on to the gravel to be the aggregate will it all mix and disperse properly and flow through the gaps? I don’t think it would.