LPG oven requires 37 mbar gas pressure

Went to look at an existing installation yesterday for a family friend who has just moved to another house. Has a Bompani 900 gas oven, rating plate says operating pressure 37mbar (3.7 kpa). On an LPG system with an Infinity 26 califont requiring 2.75 kpa. Fujikoh non adjustable Auto changeover regulator set at 2.75 kpa. Hob burners on oven working no worries at this pressure, flame picture is fine, no evidence of under gassing, but oven burner and grill burner absolutely hopeless. (2hrs to bake a cake).Anybody come across these cookers before? and what's the best way to overcome problem? ie. Can I just fit larger injectors and adjust primary air, or do I have to increase main g.p and fit secondary reg to califont? Any advice would be appreciated. Cheers