Normally when I boot my PC my router and modem are already powered on, KDE starts up, sees the wired connection and everything works just fine.

However if I forget to switch on my router and modem until after KDE has loaded the network manager refuses to see the wired connection.

It's more an annoyance than a problem but was never an issue with KDE3.

Any thoughts?

Kubuntu shipped a pre-beta version of Network Management ( KDE 4 Plasma applet ) unfortunately which still has some issues, so you may wish to try a newer version from a PPA, or use an alternative network manager such as Wicd.