Thanks guys. Putting a figure in the WAN MTU sorted it. I didn't think that was necessary as it is in the optional(?) ISP special requirements section.
Just set up a new AC86U with 384.19. All seems to running fine except when I try to set a DNS manually in the WAN tab I get "Please enter a value between 576 to 9000". Entering a number tells me that it is not IP address.
Does anyone have any ideas?