Upgraded to El Capitan on route to High Sierra, now struggling to update apps

Update Unavailable with This Apple ID
This update is not available for this Apple ID either because it was bought by a different user or the item was refunded or cancelled.
I'm at a loss. It is my Apple ID and definitely not bought by a different user though I have had to change my password from the original one I had. As for refunds or cancellations the only thing that springs to mind is that I had to take my Mac in for a repair. It had completely stopped working due to a mother board issue, but the work was done for free within the 6-year rule. This probably has no bearing whatsoever, but who knows....
Any thoughts or help would be very much appreciated; I have no idea where to go next on this one.