Thank you Colin for your advice.
I used to think that unlike big sensors, a small CCD like ICX814 is easy for imaging and more forgiving in terms of perfect alignment. Well, tiny pixels highlight any misalignments very well!
After recent investigations I realised that I may need to move the Riccardi reducer in by about 1-2mm to flatten the field. I agree, there is a tilt with both units and Catalin very kindly offered to make an adjustable adapter that will help me to tune in spacing and tilt. Also, moving the flattener in by 1mm will make the field even flatter.