Advanced sampling and testing techniques are applied to assess the mechanical behaviour of a stiff, silt-and-clay dominate, stony, low plasticity UK till whose genesis and post-depositional history imparted complex profiles of yielding pressures and shear strengths that differ considerably from those of K0 consolidated waterborne sediments. It is shown that the till reaches the limits of its elastic behaviour and undergoes Y1 yielding at very small strains, after which stiffness is non-linear. Tests reach stable critical-state failures at strains exceeding 25%. The Y1 yield surface is kinematic and, once engaged, travels with the effective stress path, as does a second Y2 surface. Stiffnesses vary with mean effective stress raised to fractional exponent ≈0.5. The till displays far more marked sample-size dependency and anisotropy in stiffness than in shear strength. It also shows significantly strain-rate dependent behaviour and displays an isotach response. The main features of the till’s shear strength and dilatancy behaviour can be synthesised within a critical state based interpretive framework that brings field and in-situ testing outcomes together with laboratory experiments conducted over a wide range of stress levels and over-consolidation ratios, considering both natural and reconstituted specimens.