Broadband router with failover facility - recommendations?

I'm after a broadband router (i.e. not ADSL, the 'internet' side will
be an ethernet connection) which has some sort of failover connection
that will enable me to use my exisitng ISDN connection as a backup if
the broadband fails.

My existing ISDN connection is via a router so ideally I need a router
with two 'internet' side ethernet connections. A serial failover
connection is much less convenient because of all the hassle that
would be involved in configuring the connection (already done, and
easy in the existing ISDN router).

Can anyone recommend a router that could do what I want, or maybe
suggest alternative solutions. I know I could leave my existing ISDN
router connected to the LAN and set it as an alternative default route
but this would involve reconfiguration of all computers on the LAN so
they see the ISDN router as a secondary connection.

The Vigor2900i can do what I want but is expensive, I'd really prefer
to keep the price below £100.

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Chris Green