Since six decades the understanding of interplanetary propagation of solar flare accelerated, energetic charged particles in the inner heliosphere has not yet achieved sufficient closure. The essential mechanisms acting on these charged particles, which perform helical orbits along the large-scale magnetic field lines as probes, have already been identified. However, in particular the impact of the three-dimensional, small-scale magnetic fluctuations on the particles' trajectories has not yet been fully understood. These superimposed disturbances are expected to interact with the charges via resonance principle – leading to both field-aligned scattering and diffusive cross-field displacements of the particles' guiding center. Since numerical solutions and known theoretical formulations have failed to verify the measurements so far, Ruffolo's equation – which is a special formulation of the Fokker-Planck equation – is applied to take account of the current knowledge about field-parallel transport; The partial differential equation is extended to a two-dimensional model within the ecliptic plane by a spatial diffusion term perpendicular to the field. We assume an idealized Archimedean field neither with polarity changes nor large-scale disturbances such as traveling magneto-hydrodynamic shock waves or magnetic clouds. The transport equation is solved numerically by finite differences. For typical ratios of perpendicular to parallel diffusion coefficient as deduced from theory, various fits have been found in good agreement with multi-spacecraft measurements. Some events and the occurrence of observed sudden flux drop-outs suggest that scattering on magnetic field irregularities significantly varies from one flux tube to another. In addition to the already existing, but sparse set of particle observations at different positions, once the current solar minimum has passed by, a new set will be available from the recently launched STEREO satellites.