Looking at your photo and image of a new one:
1) Switch off water at Stop Cock
2) Open cold tap to drop water pressure.
3) Put bucket under it.
4) undo the small screw that locks the cartridge into place
5) Twist it 90deg (I think clockwise when looking from above as it looks as if there is a yellow plastic knob to stop it going anti clockwise.
6) Drop it down about 3cm.
Issue you have is that that wood is in the way stopping it from dropping.
Lots more answers and help here on the Amazon Q&A:
https://www.amazon.co.uk/BWT-AC002400-Combi-Replacement-Cartridges/dp/B00GY2VTIU Please check that firstly you have undone the small screw that locks the cartridge into place, you should then be able to unscrew the cartridge and pull it off. If the cartridge is still stuck then you can use x2 flat head screwdrivers to gently prise it off. Occasionally the cartridge media can clog in the spigot making it harder to remove, where this happens you may want to remove the metal elbow from the fitting, place this in hot water which will dissolve any residue enabling easy fitment of the next cartridge. If you would like any further advice please call our customer services team on 01494 838100, thank you! see less
By BWT UK Marketing MANUFACTURER on 12 December 2018
Sam,
Note that I have never see this item - I am simply looking at your picture and the instructions on Amazon.
But I presume the screw is the item in middle of the following image.
Hi, tried to look at the cartridge ,think it has exploded, when I literally tiuched it, it made a noise and loads of water came spewing from a nearby pipe. I tightened up the pipe, cartridge fell away, I have someone coming tomorrow am! Oops