Does my boiler need it's gas pressure adjusting ?

My Sabre 35 Combi boiler fires up ok when i turn on the hot water tap, but the water has to run for ages before any hot water comes out.The boiler has had the gas jets changed to run on LPG and the jumper has been put in to convert it to run on LPG.
When the central heating system is fired up everything runs ok, within minutes the heating is warm & hot water is instant, could it be that the gas pressure is not set correctly.