The BT wholesale checker is the only one worth paying attention to, all the ISP's take the results from that when you put your phone number in on their website.
Not really. BT supply the ISP's with details of cab/postcode/uplift ratio when the cabs are ready to go live.
The administrive side of things (updating all the postcodes/lines on the checker) seems to lag behind the ISP's, whose interest it's in to get people connected.
I'm not sure what's wrong with what I said. Generally speaking you can usually only order FTTC after the BT wholesale checker has been updated saying your line is live. There may be the odd example like Billford's where people have managed to get it before, but I don't think that's common place.
From what I've seen, the ISP's checkers all change at the same time as the BT Wholesale checker. For instance my cab changed today to 30th June, and every ISP checker updated likewise.
Also when the BT Wholesale checker is down when they're performing updates then the checker for other ISP's is also down, so it seems to me they're just using that.
Some ISP's e.g. Aquiss even tell you the exact same text the BT checker gives you.