Firstly, I've contacted Phone Co-op again, who now tell me that some of their suppliers/wholesalers do subscribe to the IWF, and blacklist sites (thereby affecting wordpress users), but say that its up to WordPress to request their removal from IWF's list. I'm pursuing this with them.
Secondly, from the comments of rjaybee and others, it seems clear to me that my problem is different, in that my blog is working more or less normally, once I have access to it. My problem is simply not being able to get into access to it, intermittently (still).
(I have had some glitches within WordPress for a while -the 'like/comments' alert not working, and sometimes editing in full screen saves the wrong draft!- but these have been going on for several months). Like rendzeretlen I've found it very helpful to hear from others coping with this frustrating situation.
Lets hope that negotiations sort out the blacklisting mess before subscription to IWF becomes mandatory. I