Linear and nonlinear gyrokinetic simulations are performed in experimentally relevant scenarios built from a MAST case where a microtearing mode instability dominates at ion Larmor radius scale. This collisional microtearing mode instability appears only when a velocity dependent electron collision frequency is considered. Electrostatic potential fluctuations are found to provide a strong destabilisation mechanism. The sensitivity on the electron collision frequency is investigated in both linear and nonlinear simulations. While the effect of electron collision frequency is moderate in linear simulations, a strong dependence on this parameter is found in nonlinear simulations. The effect of magnetic islands generated by microtearing modes and their interaction is analysed, showing that the radial extension of the stochastic region caused by islands overlapping plays an important role in determining the saturation level of the microtearing mode driven heat flux and it is consistent with the heat flux increase observed in nonlinear simulations at low electron collision frequency values.