Yes there will be a conflict. The OR modem (at least the Huawei HG6120) is set at 192.168.1.1. I expect the ECI one is as well, as the same Home Hub is used.
IIRC the Home Hub is set to 192.168.1.254, so if you change yours to that you should be OK.
You are correct that the OR modem does act as a simple modem, although it is in fact a modem/router that can be unlocked and some even use it as a two-port router.
I suggest you do not mess with the modem itself during the first two days, as the Openreach DLM is watching it more closely than normal for stability during them.
(Edit - and stop yours allocating 192.168.1.1. But I expect you already have it set to 192.168.1.3 upwards).
My broadband basic info/help site - www.robertos.me.uk
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Edited by RobertoS (Sun 04-Nov-12 17:14:42)