So dropped into Robsons last night to discuss with the boys what was going to happen next. Craig the service manager said that the idler pully was knackered which I was already aware of and advised to change that out first as it was making a hellva racket and they wanted to mitigate any extra noise in order to make a better diagnosis but besides that they had it up on the hoist and were convinced that the big end hasn't gone.
Received a phone call about an hour ago after they replaced the idler pulley assembly and they still dont reckon the big end is gone. They cant do the rear main seal until next week so I have some time to decide whether to tell them to rip the sump off or not and inspect it but these guys seem pretty sure there is no issue.
All I can look at is the evidence, I made sure she was topped up, she was running fine and I had no oil light come on any point.