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So… With my Raspberry Pi now connected to two networks - one with the LAN port, on a closed network, and one via a wifi dongle, with internet access, both with gateways configured (because I can’t work out how to tell the router on the closed network that it doesn’t actually have internet access!..) - the Raspberry Pi prefers to use the LAN port for internet access. But, of course, this doesn’t work.

~$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         10.101.1.1      0.0.0.0         UG    202    0        0 eth0
default         www.routerlogin 0.0.0.0         UG    303    0        0 wlan0
10.101.0.0      *               255.255.0.0     U     202    0        0 eth0
192.168.1.0     *               255.255.255.0   U     303    0        0 wlan0

Note: the Metric value for the gateway entry for eth0 is lower than that for the gateway entry for wlan0.

Based on this answer on raspberrypi.stackexchange, I just needed to add a couple of lines to the end of /etc/dhcpcd.conf:

# Give eth0 a higher metric than wlan0 (default metric: 303)
interface eth0
metric 402

Note: in the answer on stackexchange, each line is terminated with a ; - this doesn’t seem to be necessary, looking at the rest of /etc/dhcpcd.conf - and indeed seems to prevent it from working.

Having made the change, restart dhcpcd (the DHCP Client Daemon) and check the results:

~$ sudo service dhcpcd restart
~$ route
Kernel IP routing table
Destination     Gateway         Genmask         Flags Metric Ref    Use Iface
default         www.routerlogin 0.0.0.0         UG    303    0        0 wlan0
default         10.101.1.1      0.0.0.0         UG    402    0        0 eth0
10.101.0.0      *               255.255.0.0     U     402    0        0 eth0
192.168.1.0     *               255.255.255.0   U     303    0        0 wlan0

There’s also an interesting clip from man dhcpcd.conf (5), which explains why dhcpcd is assigning the wireless interface the higher metric:

metric metric
        Metrics are used to prefer an interface over another one, lowest
        wins.  dhcpcd will supply a default metric of 200 +
        if_nametoindex(3).  An extra 100 will be added for wireless
        interfaces.

As a side note, there’s some interesting information about the various configuration files used for network interfaces (/etc/network/interfaces, /etc/dhcpcd.conf and /etc/wpa_supplicant/wpa_supplicant.conf), and the differences between (Raspbian) Wheezy and Jessie in this answer, here.

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