MAC addresses don't generally change unless you reassign the address to
clone a NIC to a router.

Are you sure you don't mean dynamically- allocated IP address?

yes im sure, i have to change the mac address in my router to
'00:26:54:0a:c4:10' to get it to work. If i click on the 'Clone MAC Address'
button it comes up with an address and it stops my internet connection
working.

I suspect that the Nildram DHCP lease is set to
expire after 24 hours, and your router isn't detecting the modem's new
address - have you set the WAN connection for dynamic IP addressing?

yes, i have it set on
-
Dynamic IP Address : Choose this option to obtain an IP address
automatically from your ISP. (For most Cable modem users)
-

There aren't any login details or anything on my router - I have to set it
up on my modem and hope that it knows what it's doing when its connected to
my router.

If it breaks again tonight while im asleep i'll be really annoyed.