How can I fix the safety cut-off on a Rangemaster lidded classic 90?

I have acquired a Dual-Fuel Rangemaster Classic 90 with glass lid, but no gas supply is reaching the burners. The lid has a safety mechanism that cuts off the gas supply to all burners when it is lowered. The appliance stood for some months in a garage, and the cut-off mechanism seems likely to have become stuck in the 'Off' position. How can I fix this, please?