On the SP's class base, I mainly agree with you and Dave. However, I think the SP have a grotesquely inflated self-image.
On the strike rally in London yesterday AWL comrade Jill Mountford approached Peter Taaffe to ask if they'd be debating us on Libya (the answer now seems to have shifted from a tentative yes to no - possibly this was Taaffe's spur-of-the-moment response to feeling pissed off). Two physically large male SP comrades then attempted to interpose themselves between Taaffe and Jill. One of them, a Tube worker whose name I can't remember, repeatedly told us that we are a "middle-class sect" with "no influence in the labour movement". I asked him how it was, then, that in his union our comrade Janine Booth had beaten his comrade Lewis Peacock hands down in the election to represent Tube and TfL workers on the executive - this as part of the process of building a substantial AWL group and a network around the rank-and-file bulletin Tubeworker. (Now I'm wondering if the comrade was in fact Lewis himself!) He denounced Janine and our other Tube comrades in very strong terms, as did Taaffe.
My point is that I think the SP have a tendency to behave as if they were a mass workers' party which leads the labour movement or at least a dominant left opposition in it, and dramatically downplay other tendencies' influence. They talk as if they were an elephant and we were fleas. Now, even if they were a mass workers' party, that would not require us to defer to them - what about the Communist Party from the 1940s to 1970s (which, in fact, the SP behaves more and more like)? But in reality they are very far from that, with 1,000 members maximum and perhaps a lot less.
That's also why I disagree with Dave that they generally behave more reasonably in the labour movement than the SWP. I think they are just as sectarian, but in a different way.
Theo, your other comments are interesting and I need to consider them.