The quantum tunnellings of the magnetization vector in single-domain ferromagnetic nanoparticles placed in an external magnetic field at an arbitrarily directed angle in the ZX-plane are studied theoretically. We consider magnetocrystalline anisotropies with trigonal and hexagonal crystal symmetry. By applying the instanton technique in the spin-coherent-state path-integral representation, we calculate the tunnel splittings, the tunnelling rates, and the crossover temperatures in the low-barrier limit for different angle ranges of the external magnetic field (θ_H = π/2, π/2<<θ_H<<π, and θ_H = π). Our results show that the tunnel splittings, the tunnelling rates, and the crossover temperatures depend distinctly on the orientation of the external magnetic field, which provides a possible experimental test for magnetic quantum tunnelling in nanometre-scale single-domain ferromagnets.