Thanks for the suggestion Mimesis. Reading the comment about using 'a cheap switch', I looked at the switch I was temporarily using to connect the laptop and Meridian devices.
It was a Netgear GS105E. It was a cheap switch, but looking at the Netgear webpage, a support app can be download to update it's firmware.

I did this and noticed that a second GS105E also appeared. It is a switch installed in the back of the TV cabinet in the living room.

After updating the firmware on both switches, I looked at the other available options in the app and found a Multicast page for IGMP Snooping Configuration.
It was enabled, so I disabled 'IGMP Snooping Status' and 'Block Unknown MultiCast Address'. I repeated this for the second switch.

This solved the problem and I was able to update both Meridian devices.

Anthem AVM70, 271, DSP7200x2, DSP7200.2HC, DSP5200x4, JL Audio F113,
Oppo UDP203, Panasonic UB9000, Kaleidescape Strato E, Apple TV 4K,
HDFury Vertex2, Epson LS10500, Stewart Filmscreen 110"