I have now realised that the concrete floor varies in level by 7mm over the 4 metre length of a row of base units which means that I will have to either
i) level the floor or try to adjust the level with varying thickness of tile adhesive
ii) scribe and cut the lower edge of the plinth to match the varying height of the tiled floor
iii) fit the base units and plinth and tile up to the face of the plinths.
i) seems to be a lot of hard work or a lot of adhesive/self levelling compound, ii) a bit fiddly, iii) may make it awkward to remove the plinths at some time in the future in the unlikely* event that I need to gain access. * I am cutting access holes in the back panels wherever there may be a need to get to service valves and power outlets.
Grateful for your thoughts.