Guys, after about 8 or 9 tries at this I have found the issue.
On the first tower please ensure you kill all the Risen before the tower is blown. There are 2-3 that are very hard to see (inside the tower structure), if these two aren’t killed when the tower is levelled then one or both will bug inside the rocky mound that’s left.
The way I needed to do it in the end was to run way ahead of the Tonn and the Marmox and really make sure it’s clear before they get there.
It’s a silly bug, but every time this bug happened progress was halted after clearing the beach, once clear it work’s.
Hope this helps.