Thank you both @Mark and @Avionic for your replies.
I can confirm I am using the custom 146 Simbrief profile, though as has been mentioned, the problem is unlikely to be with the profiles but more likely with the actual fuel burn rates in the simulator.
Using my latest 146 flight as an example, the estimated Enroute Burn was 3930kg for a Route Distance of 614 nm (Estimated TOW 40,671 kg). The actual recorded burn was 2637kg.
I am happy to provide further flight plans, figures etc and raise a support ticket with those details but I get the impression there is something more fundamental going on here. Simulator settings? Other possible conflicting addons such as GSX? Etc.
Thank you both again.