In this paper, we propose a parasitic patch-based method to improve the power transfer efficiency of radiative wireless power transfer (WPT) systems involving implanted biomedical devices. The proposed WPT system uses circularly polarised antennas, having a four-element sequentially rotated antenna array, as the Tx and a miniaturized circularly polarized antenna implanted inside a body mimicking gel as the Rx with a parasitic patch placed in close vicinity. The design guidelines for the dimensions of the parasitic patch and its distance from the implanted antenna are discussed in-depth based on rigorous parametric studies and analysis. The proposed parasitic patch-based method improves the implantable antenna gain by 7.83 dBic, thereby increasing the power transfer efficiency at 2.4 GHz operating frequency. Improvement in power transfer is experimentally demonstrated with the help of a low-cost experimental setup having a software-defined radio (SDR) and a spectrum analyzer. Further to analyze the effect of interfering signals on the performance of the WPT system, power received by the implantable antenna is measured in presence of a secondary transmitter.