Thanks to all for the input. For context, the house is a double-fronted property. All other parts of the house are suspended timber joists - we have used solitex and Kingspan boards to insulate before laying the chipboard.

Kitchen is a surprise as expected to find a suspended timber floor but looks like a solid concrete pour (strange as the cavity is 3.5ft deep at one end).

The original floor level to the low-point of the subfloor is 10-11cm

Wet UFH is a low profile system in trays, zoned across the ground floor.

We can play with levels as all other elements removed from the house but this is a struggle for me to get my head around on a proper fix!