Currently the Dial-In end is 3G; The Dial-out ends are ASDL
But it's LAN to LAN, so we could easlity set it up the other way round.

Voda 3G in the UK uses a proxy between you & the Internet. Thats why you get a 10.x.x.x address instead of a more familiar Internet IP address. Remember that 10.x.x.x, 172.16.x.x & 192.168.x.x are all for internal networks only. So the proxy will filter out all sorts of stuff that Voda don't want floating around thier networks. I have got SSL, PPTP to work without an issue on occasion but have never got a PSK IPsec to work properly. The cert type seems to work ok... Must be something to do with the endpoint IP or proxy in the middle.

I think the Vodafone user forum has some info on how you might be able to get VPN operational.

I think 'lozstlouis' has it right. That is basicaly what I found when trying to get it to work with a few protocols and in the end gave up

Yes, I gave up too.
I have been told that the THREE network will give you a genuine fixed IP, but we do not have reception here so I didn't try them either.

None of my Three mobile broadband SIMs have either a public IP or fixed IP.