Thanks all for replies on this and, regarding the key solution
@dblhelix I agree with you but as mentioned by
@Alaskan I was offered the option to get back on the road that day vs potentially 'months' for backorders to wait for Abus to send back a keyed alike solution, so I took the option that got me back on the road quicker. If that was the only problem that occurred I would count myself lucky, but this belt issue is considerably more time and effort.
From the above, I feel fairly certain that belt tightness is the issue, but unfortunately I don't have the space or tools to properly make this change at present, so back to the bike shop I will need to go. They're currently late at backlog on repairs, so was trying to avoid being off the road for too long if I can help it.