Just a quick update to this, decided to replace both rear adjustable shocks with the standard items. Spoke to Lee at Autojapspares who was really helpful and had a set in the post the same day. Got them swapped out at the local garage who just disconnected the sleeping and cable tied it out of the way. Working a treat and no warning lights on the dash. £120/pair instead of £1000+ for one! Pity the adjustable units aren't readily available at a decent price anymore but no real loss.
Kept the offside adjustable which is still in really good condition, will pop out on eBay at some point to hopefully recoup the cost 👍
Thanks again to those who replied and hope this helps anyone in the future.