There's just one NCB Sentinel left at Amazon at present. (Poor thing must be feeling lonely!). Of course I succumbed; I also bought one of the Tarmac ones after I posted earlier but being a cheapskate, I opted for free delivery - it'll be here sooner or later :-).
On the availibility question, I saw yesterday that all the Sentinel variants were being advertised for sale on the Hornby Shop site, although the Tarmac one is back on "stock notification" already! I haven't really kept an eye on their stock levels, but if this stock is the result of a new batch (post Xmas), then the silence from the usual retailers is deafening, unless Hornby is embargoing stock until they've sold some on their own account. We may have been lucky with Amazon, they sometimes jump the gun, usually with popular books, but I'm not complaining!