I brought a HX510 in person from Digitech Electronics in Hounslow (www.digitechelectronics.com) for £294.99. I did ask about the HX710 and was told that Sony does not have any in their warehouse, and are awaiting the next arrival from their factory. Incidentally, the HX510 is great: albeit it does have a smaller hard drive and an analogue tuner when compared to the HX710.
On the Sony Dealers Website, the HXD-710 is showing as "not available until 20th February" and the HXD-910 simply says "contact Sony" with a red warning symbol next to it!
I've had an HXD-910 on order from Gultronics since November - Looks like I've got a lot longer to wait....