Identical MAC Addresses on Network

Use a router to create a second subnet for your heatpump (or alternatively, for the Wi-Fi devices). MAC addresses are only relevant within the same L2 network they're on.

If you insist on having everything on a single subnet, the only other option which could help is a switch/bridge with "layer-2 NAT" function, placed like a firewall between the heatpump and the rest of the network. For example a Linux software bridge could probably achieve this via 'ebtables', and most Mikrotik RouterOS devices have L2 NAT under '/interface bridge nat' as well.

Without doing either of the above, trying to keep both devices in the same subnet will be nearly impossible, as every single switch or Wi-Fi access point will be constantly changing its mind about where to deliver the packets, and whenever they get delivered to the wrong device it will kill the unrecognized TCP connection (which is normal behavior).

(Note: Even with two subnets, if you try to do this using VLANs, it can still cause trouble if the network switches don't have IVL...)

If they really have the same MAC (which is weird and shouldn't happen, but it isn't unheard of) they conflict with each other at the ethernet level.
Your switches/routers and AP's (and whatever else connected to your LAN) can't distinguish between those devices (as the MAC is the unique ID at ethernet level) so traffic intended for them gets mixed up.
(MAC filtering doesn't have anything to do with this, as you already found out.)

If you can't change the MAC at device level the ONLY solution is to put them in separate LAN's or VLAN's. Unfortunately most SOHO equipment doesn't have VLAN capability.

If your router/AP allows the use of a Guest WIFI that has a different IP-range than the main network try to put the Nintento Switch on that Guest Wifi. With a little bit of luck that Guest Wifi is also a separate VLAN and that would fix the problem.

If the Switch is still under warranty it might be possible to switch it at the store for another one which should have a different MAC.

All sorted, Nibe UK passed the problem to their head of Technical and they arranged for a new mainboard to be sent over and fitted, all fine now :) for whatever reason the original board had the same MAC as the Nintendo Switch, the new one doesn't - possibly the "Heart of Gold" was passing over when I installed the kit :) Thank you all for your help and suggestions.