Not easy, you'll have to remove the turbo first iirc, just buy branded gasket, i used THIS

it depends on the brand, Mahle or Mann have the o-ring attached and most aftermarkets too e.g https://www.jgs4x4.co.uk/defender-discovery-2-td5-engine-oil-rotor-filter/

Check if you have power in C0660 pins 1, 3 and 13 against pin 11(earth) with ignition on and if you get +12V in all of them replace the BCU, disconnect the battery first then after replacement and battery reconnection turn ignition on(dont try to...

This means that the electronic side is working cos the signal gets there... if the latch mechanism wasnt bran new but some scrapyard part send it back and ask for refund cos that's the problem...if the latch is new well speak again but not untill...