Your DS is an adult but reading your initial post this is what jumped out at me:
"The last few years DS has become considerably worse He’s actually become like it to DH and I over the last year or so too."
"He became disdainful of DH and I and wouldn’t listen, telling us we didn’t know what we were talking about etc. Well the proof was in the pudding when he failed last August."
"we insisted he got a PT job"
"he was offered a brilliant apprenticeship in the summer and we more or less insisted he took it - he had absolutely no plans to go to university so we felt that this was the best option for him."
"It feels like he’s punishing us for something (probably as we got firm over the AS levels) "
"but he has everything! A lovely home, stable family etc. He’s 18 and wants to be treated like an adult but he barely acts like one."
"DD is here and is bored so asked if she could go on his Xbox to be able to game with her friend. He text almost immediately to me to demand that she gets off it."
Firstly he is a normal teenager - hormones and all, trying to find out who he is as an individual. Which is very difficult when you become an adult and have to live at home.
Next - you are treating him like a child, insisting he gets a part-time job, insisiting he takes the apprentaship, what about what he wants?
However on the flip side he need to respect those around him. But how can he when you treat him like a little boy. You have just done it again by giving his little sister permission to use his personal things without his permission..... you do not respect his boundaries, how can he respect yours.
Time to sit down and have adult conversations about adult responsibilities and respect.
You need to apologise for the xbox error, and use that as a starting point for asking what he considers reasonable boundaries. Then you need to have pre-agreed with your husband what is reasonable boundaries for the family home. Basic politeness is essential,, but going above and being forced to be extra nice is not, sharing is optional on both sides. whether that be a wifi code or xbox or car or anything above essentials (bathroom, kitchen and washing machine access!)
You cannot expect to get respect from your son unless you give him respect (including his own personal belongings)