"When in third, and downshifting into second, its extremely jerky sometimes so I have started doing a single clutch downshift and that has helped alot...but its still relatively jerky."
Yeah, single-clutch downshift is the way to go. Double-clutching not required. You should not be using the transmission for slowing anyway. Downshift when you need the lower gear in order to accelerate. Use the brakes for slowing. Brake pads are cheaper than transmission repairs.
Remember, the lower gears (1st and 2nd) are comparatively steep, so try waiting till you have slowed more than for the higher gears before shifting.
Make sure you have rev-matched before releasing the clutch.
Practise makes perfect.