Trace amounts of tritium gas have been injected in short puffs into JET ELMy H-modes with a wide range of deuterium gas-fuelling rates. Analysis of the subsequent time evolution of the neutron profile and extraction of the particle transport coefficients have allowed us to distinguish between broad classes of mechanism which have been suggested as explanations for the H-mode density limit. The high penetration probability (~20%) and rapid transport (~tE ) of fuel ions are shown to be only weakly influenced by strong gas fuelling - hence mixture control is possible even when the total electron content is clamped.