We discuss the phase and group modal birefringence in photonic crystal fibres (PCFs) with an elongated core. Owing to large form birefringence, these two types of birefringence in such PCFs may not only have opposite signs, but their absolute value can also differ by several orders of magnitude. We also show that PCFs offer the unique possibility of having a large phase birefringence and a negligible polarization mode dispersion at the same time. Using a fully vectorial mode solver, we show how these parameters can be tailored by a proper choice of the geometry of the PCF. We demonstrate both numerically and experimentally the strong wavelength dependence of phase and group modal birefringence in triple-defect photonic crystal fibres (PCFs).