My Actifry has just developed a problem with the paddle turning but juddering and making a loud clicking sound

If this is a older model , then the internal cone that fits into the paddle is broken or not fitted correctly, or remove paddle to check that motor is still turning, if either of this is at fault its difficult to get spares E Bay best bet, also not easy to take apart,

I have just looked for a paddle for my Actifry and both the above sites are very expensive i got mine from www.kgasupplies.co.uk and it was far cheaper
Kga Supplies Spare Parts Accessories For Home Appliance

Replace the paddle motor. Spares are redily available on Amazon

Paddle won't rotate

You have just two options:-

Return the unit to Tefal for a repair under warranty, or claim under a separate product warranty.

If you have no warranty then your only fix is to replace it with another ActiFry. But as they're notoriously unreliable you might choose not to.

There is almost nothing user-fixable on the ActiFry and it has not been designed to be repaired. They have about a 3 year maximum lifespan assuming an "average" domestic usage cycle, and far less if used every day.

Purchase a SS-992500 - The Transmission Shaft & Motor has defaulted. We have repaired many of these.
Depending on your engineering ability you may be able to do this yourself.
However, if you needa ny further help on the instruction for repair or purchase of part dont hesitate to contact us at:
www.wescoelec.co.uk

Actifry Paddle and Actifry Filter are available here: http://www.actifry-parts.co.uk/

3 star key screws on the base of the unit, loosen the power cable from its clips then with a flat head screwdriver separate the 2 plastic parts starting from the back taking care not to damage the fan motor.
I managed to get my motor working with a bit of a clean and making sure the motor and paddle were aligned or a new motor swap involves 2 screws and push on connectors, no soldering.

You can get spares for an Actifry at Wesco Electrical, whose link I found buried deep in Tefal's own site. Lid is available here: http://www.werl.co.uk/lidcover-complete-ss991271-p-3106.html (£21.50) and paddles here: http://www.werl.co.uk/mixing-blade-seal-ss990596-p-2203.html (£18.21). I've got a lid on order, as the two tabs have started to break under the strain of the spring holding the lid open.