Thanks. He is someone who has been featured in bike mags for builds he has done, so he has a good rep.
He says he thinks someone has been fiddling with the bike, as the chain has stretched due to being overtightened. As far as I understand, he reckons the tensioner has failed, so someone has bodged trying to tighten the camchain, which has stretched it and caused some damage to surrounding components.
He said that without further expensive investigation, it's impossible to tell what damage has been done, so he quoted 12 to 1500 pounds to change the whole camchain system, as he said he didn't want to do a half job.
He also spent his own time off the click looking for a motor to swap if I didn't want to fix this one.
I kinda have to trust him, any mechanical work is done with an element of trust right? I'm not very good with the spanners, and I told him about all the comments I received here, but he's adamant it's in need of major work.
I asked if I could carry on riding, he said if it gets louder then stop, and don't ride it too aggressively.
Maybe I could look into resetting the tensioner as you say.
It's a lovely bike, with great character, and I appreciate the kindness of you guys replying. Thanks.