We currently have Webb,Royal Brierley and Stevens and Williams.......................
Patrick, would you count Whitefriars and Powell as two companies? If so then you're correct in counting this as 3

it is "well known" amongst WF aficionados that a vase like this, or a very similar design, was also made by Webb
.....and outside W/f's circles too Leni

I'm getting hints of the appalling eBayworld habit of attributing anything unrecognised, unmarked, and handmade to Webb or Stevens & Williams.
Bernard, firstly this is because many of the techniques and patterns used by Powell/Whitefriars were made by those two companies - for reasons well outlined by Ivo. Secondly, isn't it good that those folks are at least trying to think outside the very limited 'Whitefriars' box?
Finally, my vote goes for Stevens & Williams........or............Webb

Nigel