But am I right in thinking that the MAC addresses shown as connected do not change?
Yes. The MAC Address is globally unique (at least in theory)* and hardware coded to the network card.
Some network cards / adapters do have the ability to be software overwritten - most notably these days in cable / adsl routers where the MAC address can be overwritten with the MAC address of the PC used to initially connect to the modem. But it's not usually an issue.
SD
* I remember receiving a batch of PCMCIA network cards (about 25 IIRC) at work about 10 years ago - and they all had the same MAC address! Caused considerable consternation and confusion!