Haven’t used for a year, fitted, very difficult to get it moving at first but it goes down with a bit of a jump on the saddle. Then once weight is taken off it rises slowly back up.

Is this something I can sort myself, or is it an expensive service?

Check the air pressure first – Schroeder under the but where the seat rails go. It needs to be 250psi.

The valve might be right in the bottom of the post…

Check the speed of return adjuster on the lever
And check your seat post collar isn’t overly tight

Probably wasn’t clear, it isn’t holding the position it is set to, ip the post gradually rises if I’m not sat on it, even without the lever being used. If I put weight on it, it stays at the height, so it is rideable, but not how it was

In need of a bleed, sounds like it’s not properly opening/closing.

I’d say the seals have gone and needs a seal kit/overhaul, mine did the same after 2 years 3 months sent to TF tuned sorted for £95ish