This paper presents a numerical study of single-electron resonant tunnelling (RT) assisted by a few ionised donors in a laterally-confined resonant tunnelling diode (LCRTD). The 3D multi-mode S-matrix simulation is performed newly introducing the scattering potential of discrete impurities. With a few ionised donors being placed, the calculated energy-dependence of the total transmission rate shows new resonances which are donor-configuration dependent. Visualised electron probability density reveals that these resonances originate in RT via single-donor-induced localised states. The I-V characteristics show current steps of order 0.1 nA per donor before the main current peak, which is quantitatively in good agreement with the experimental results.