I'm after an ATA that can have one BT line plugged into it and one
analogue phone, and manage at least one SIP account.

Nothing out of the ordinary there, I guess, however I want the ability
to be able to put an incoming caller on the BT line "on hold", make a
call on the SIP line, and then transfer the call from the BT line to the
SIP line (the ATA acting as a bridge or mini PBX in this instance)

(The SIP account will be on a virtual PBX and other phones connected to
the same vPBX and we want to transfer the call to one of these phones)

And yes, I know this will tie-up the BT line and maybe even the local
phone and SIP line for the duration of the call, but that's not an issue
here...

Looking at the SPA3102, it seems like this will do the trick, but can
anyone verify this for me? (Or suggest a better alternative!)

Thanks,

Gordon