Just in case this is every of any use to anyone else, I thought I would report back.
We are using 4mb i-Choice, and so far it is working fine. The supplied router is the Speedtouch 585, and after a bit of playing around we were able to get this in to Bridging mode, where it gives the IP address to the firewall behind it (currently an Endian -
www.endian.com) which is set up to connect with DHCP.
There is something strange going on, presumably with NAT, which has made the IPSEC tunnel more difficult to configure. The static IP that Cyta supply is a real IP beginning 213. However the IP address actually supplied to the firewall is a private IP beginning 10. Going to
www.whatismyip.com shows the 213. IP, and I am able to ping that - but because the firewall does not have that IP itself, the IPSEC needed to be configured differently with certificates and only the Cypriot end initialising.
Thanks again for this invaluable website.