On SW site I can't see the chicken breast slices, but found the wafer thin chicken:
Wafer Thin Chicken Slices, chilled , per 100g 1 syn on Red and EE, 6 syns on Green
Wafer Thin Chicken, chilled , 70g pack Free on Red and EE, 3½ on Green
Not sure what the difference is, but think you have to syn yours
Doesn't look like there is much of a difference. The pack size varies. 70g pack is free by your calculations but the other is 1syn per 100g. That could be rounded up if you catch my drift and may also be free at a 70g portion? So it would depend upon how much you ate at one sitting.
Zooming in shows 3 slices is 25kcal. That's without a free food allowance so 3 slices would be well under 1syn.