I haven't done this with this particular modem but I would try the following order.
First connect the laptop to the WAG54, it should then get a local LAN IP, so you can log into its setup pages to disable the DHCP server and then save this configuration.
Then power everything down, connect the ntl modem to the WAG by Ethernet, disconnect the Ethernet between laptop and WAG54.
Power the ntl modem up, when lights have settled power up the WAG and the laptop. The laptop should then get the NTL supplied IP address and internet should work.
NB the NTL modem will only give the first device to connect an IP address (laptop wired will be different device to laptop wireless), when the laptop has the NTL supplied IP address you will probably not be able to access the modem. To do so you either reset the WAG to defaults or assign you PC a static IP address in the same range as the WAG is 192.168.1.100 netmask 255.255.255.0.
If this doesn't work please let us know exactly which version of WAG54 you have. Linksys provide web screen simulators so there may be more settings can be tried ie
http://www.linksysdata.com/ui/WAG54G/v1/1.00.2/