As ever with these types of queries, trying to visualise what you are doing

1st Scenario

Which bit has the CE mark - the ladder or the platform? If the platform has the CE mark, the 'paperwork' should say what the intended use of the platoform is. I assume this is a platform that you have already purchased from elsewhere. I don't think there is anything inherently wrong with what you are doing - just did the plaform designer anticpate the platform being used to support a ladder from?

Will it/is it sufficiently stable/rigid to accept being loaded in this way? Will it over turn with a sideways loading on it? How do you stop the foot of the ladder slipping and falling? As well as falling off the ladder, how much further would the person fall? i.e. how high is the platform working surface.

2nd Scenario

If you are building a platform, and hence designing the platform then you take on designers duty under CDM. Of which it may be necessary to justify your platform design in terms of structural calculations and testing.

What is the platform made of? Steel - welded sections etc with flat plate? What load do you intend it to carry? How many people will work on it/is it intended to carry. Is this a self supported platform on its own legs? How do you intend to ensure it is stable? Won't over turn. How will people access the platform work area? Internal or external steps or  ladder?

Is the platform a cantilever attached to another structure? Again how do you show the connecting structural joints and attachment point are strong enough for the anticipated load. 

In either case if the platform is steel, overloading probably isn't a major concern for a small platform, but buckling might be a risk. 

Steel tubes/plate etc will have data available for it loading specification/permitted distributed loads etc if you ask the supplier.

As ever. more information required.

Certainly if the platform failed, it would be an obvious question that you would be asked - was it strong enough?

Long gone are the days when you could knock something up in the factory workshop and just use it.