Vodafone NEVER replied to me on this issue, so I contacted Spark and their reply from a "Digital Expert" was thus:
Thanks for your patience while I looked into this.
I've spoken to our Wireless broadband team and unfortunately it looks like IPsec tunnelling would not work with Wireless broadband.
There you have it... no reason, just a flat NO from Spark.
I've contacted them with a request to specify whether it is just their modem or the carrier service that is preventing the possibility. Will post when I hear back... IF I hear back. If ut's just the modem I'll probably get the D-Link DWR-925 or preferably something from Draytek.