The view has been taken previously that all parts of a deck need to be 300mm or less above ground level - at all points. This approach means decks which are higher than 300mm at some points would not be PD. Some councils apply this approach and it has been taken by Inspectors at appeal- see this decision-
https://acp.planninginspectorate.gov.uk/ViewCase.aspx?caseid=2175602
Quote-
Paragraph I of Part 1 states that ‘for the purposes of Part 1 – “raised” in relation to a platform means a platform with a height greater than 300mm’. That statement is not qualified or linked back to Article 1(3). I interpret paragraphs E.1(g) and I as meaning that a platform would not be PD if its height would exceed 300mm at any point in relation to adjoining ground. Adding weight to that assessment, Article 1(3) requires consideration of the context. Part 1 of the GPDO was amended in 2008 to, amongst other matters, expressly remove raised platforms from PD. It would defeat the purpose of inserting E.1(g) if platforms could be constructed to any height whatsoever, in order to level sloping land, so long as they are only 300mm above the highest part of the adjacent surface.
Other decisions take the contrary view- it's not really pinned down for certain in the legislation or the technical guidance so remains up for discussion (despite this restriction coming into force in 2008!)
So far as I can see from a quick check on the Planning Jungle site, this is one of 4 appeal cases concerning the height of decking relative to natural ground level.
The inspector in this one is in a minority, as the other three supports the OPs case. I think the 'context' argument the inspector brings in is spurious, as I can't see that qualification in the pd rules.
This is the sort of problem which really needs a judicial ruling to produce some certainty.
In the meantime - and as the decking is already there - the OP is probably best leaving matters with the council, to see if they would be willing to take the risk of Enforcement.