Good heavens. Do I sense rapprochement?

IMHO the problem with that model is that Labour are at present not materially left of centre and arguably rid of centre (sorry Red Ed, but you disappoint me) whereas the Con-Dems are a lot right of centre, the conservatives alone very right of centre, and UKIP pretty much BNP-lite.

Socialist Labour Party and SWP etc are not effectively present in the model.

A left unity would give a clear alternative to the present right (and it seems to me very right) wing conservatives and coalition.   If viewed in that light it could leave Labour occupying the middle ground pretty much alone.