Hi there - I'm only just getting to grips with this myself, but I've booked to have a fixed towbar fitted by a company called The Tow Bar Group - they told me their suppliers don't do a detachable option for this Alphard anymore. (Their list of suppliers is Witter, Westfalia, Tow Trust, PCT, Brink, PF Jones, Oris, in case it helps to short-cut anyone else's search.) I tried a few other places and they either didn't have anything for the Alphard (Towbar Express), or were a bit more expensive (uk-towbars.net).
If you could fit it yourself, you can buy the PCT towbar from Towsure (they won't fit it), and I think it works out cheaper than buying it from PCT directly - but I might be wrong on that.
That's pretty much everything I know... Good luck!