Yeah what you need is a pressure washer the type you would use for cleaning patios etc.they make a drain cleaning attachment which is like a 10m length of pipe which you feed down the gully and into the pipe.
May be too late in providing you with a reply but others may be able to use this,
9 times out of 10 the gully will block with grease and soap reside on the bend of the trap, by using a 4 or 6" plunger disc on a rod and plunging the gully will normally clear the blockage, run hot water down at the same time to refill the gully, do this for a few minutes to ensure you get the grease out of the trap.
If you have a manhole then check to make sure large sections of grease do not block up the drain.
plenty of hot water and plunging will claer any blocked gully.
The drainman keeps you following so you can keep going