Sounds like the sort of thing a decent general garage should be able to find, but you say you have had it checked by an independant expert .......
That's what I'd hoped and thought.
My opinion of main-dealer technical skills (or, more accurately, what the master tech is allowed to do by the service manager looking over their shoulder) was already rock-bottom and this has confirmed it. Truthfully, I've never been to a main dealer (inc. Ford, VW, Audi, Alfa, Peugeot, Saab, Volvo and Honda) who can do anything of any worth mechanically. Service, MoT, basic bolt-on, bolt-off stuff is all okay. Give 'em something outside the norm and they all just shrug and usher you out of the service reception.
It really isn't a question of cash; I'll pay for workshop time, test-drives, diagnostics etc etc. Whatever they want, I'll do it! I just want a decent mechanic to tell me what's up and fix it. I think a trip down to NJS in Pershore is in order. I'd go to Jamie at the Alfa Workshop but just can't find the time.
EDIT: The independent assessor did find worn CV joints, which have (allegedly) been replaced by the main dealer now, but these turned out to be unrelated to / not the main cause of the "thump" noise and I didn't want to pay him again because he was assessing the car on my driveway. By the time I've taken it to his workshop, I might as well drive it to an Alfa specialist.