Shaun,
The router can easily be configured to fall over to the IDSN in the event of a leased line failure.
You would need to check the suitability of the WAN interface card to match your particular LL circuit, in the UK they normally present 2mb as X21 as you specified in your original post. The interface card to use in this case is a WIC-1T, and a CAB-X21 interface cable.
I've configured lost of 1600 series routers to do this, its unfortunate that the 800 series only have 1 WAN interface.
To make this work effectively I would recommend running EIGRP across the WAN LL. Point a floating static over the ISDN, then if the LL fails, the ip route entries drop out of the routing table, the router then uses the static via the ISDN.
something like:
Int s0
ip addr 10.255.1.1 255.255.255.252
int bri0
encap ppp
no ip addr
ppp auth chap
dialer pool-member 1
int d1
ip addr 10.255.2.1 255.255.255.252
encap ppp
ppp auth chap
dialer string remote-ISDN-No
dialer remote-name REMOTE-ROUTER-NAME
dialer-group 1
dialer-list 1 protocol ip permit
router eigrp 100
network 10.0.0.0
passive-interface d1
ip route 0.0.0.0 0.0.0.0 10.255.2.2 200
Andy