The exam questions from my end are:
1) Is a '94 Disco's factory alarm/immobiliser still a reasonable level of protection, or can thieves defeat it easily nowadays?
2) Will an aftermarket Alarm/Immobiliser clash with the 300TDI's factory Alarm/Immobiliser,
3) Can the old and new security systems co-exist happily, or does the old one need to be disabled, removed or bypassed in some way?

The mechanic who was looking at fitting my new Viper alarm reckons that there may be fun and games trying to install the new system.

I've seen some useful gen on this thread:
http://www.landyzone.co.uk/lz/f8/300-tdi-alarm-circuit-diagram-required-please-37559-2.html
[particularly the post done on 6th-February-2007, 19:43]

Perhaps this on eBay could be of use?:
DISCOVERY 300 TDI. ALARM / IMMOBILISER SPIDER BYPASS

DISCOVERY 300 TDI. ALARM / IMMOBILISER SPIDER BYPASS | eBay UK

Anyone got any experience of alarm issues?