I already have one Dial-In LAN to LAN profile between two sites. My Main site accepting dial-In and my (client site) dialling out to it.

Having switched to Virgin Mdia only my Main site has a static iP hence the above setup works well (whenever client dynamic Ip's are changed)

I therefore want to switch 2 other sites to this mode of operation (as they have dynamic IPs as clients) - my problem is that whenever I try to create a Dial-In profile the option id greyed out and therefore in-selectable.

Am I missing something really obvious?

Because your dial in routers have a dynamic IP address, you need to use Peer ID.

Have a look here;

https://www.draytek.com/en/faq/faq-vpn/vpn.lan-to-lan/vigor-router-to-vigor-router-ipsec-aggressive-mode/