ICM telephone poll poser

Thanks to Xabi on the previous thread for letting me know that ICM are conducting a referendum poll for the Guardian - and unlike the firm's series for Scotland on Sunday, this one is being done by telephone rather than online.  Believe it or not, this will be the first national telephone poll of the entire campaign to be conducted by a company other than Ipsos-Mori, who of course have tended to produce much more No-friendly results than the online pollsters, including ICM themselves.  It's unclear how much of this divergence is due to the telephone factor itself (and in particular the apparent landline-only nature of the polls), and how much is due to other distinctive features of Ipsos-Mori's methodology, such as failure to weight by past vote recall.

This development poses a problem for this blog's Poll of Polls, which uses the most recent poll from each firm, meaning that an ICM telephone poll will be replacing an ICM online poll in the sample.  That could well distort the trend, because there's a good chance that telephone polling will produce different results (my guess is that they'll be more No-friendly, but there's no way of knowing that for sure).  The alternative would be to treat "ICM (online)" and "ICM (telephone)" as two separate firms for the purposes of the Poll of Polls - but that might give too much weight to a single firm, particularly given that they'll presumably be using exactly the same methodology other than data collection.

Any thoughts?  For the sake of transparency, it would probably be best to make a firm decision well before we actually know what ICM are showing.