Usually the built in ovens are rigid fix pipes not flexible,so open the oven door and grill doors and you should see screws into the units.Take these out.Maybe one of the top units has a removable panel which will give you access to the gas supply.You should find a thumbpiece tap on the supply so you can isolate the appliance,but it may be easier just turning off the mains.That way if you do have to pull and tug a bit you arent going to be too worried.Once the gas union is undone the oven should slide out so you can gain access to the chains.
Tony S
January 2009