Don't know if I can be of help here but when you download Flashtool, if I remember right there is a 64bit and 32bit in the package. I am running Windows 8 64 bit and that one didn't work for me for some reason but the normal one seemed to work fine. If I understand your question right, I think it uses the drivers that install from the phone / Sony Companion.
Unfortunately I doubt you will see 4.4 for some time yet, if it has just started rolling out I believe UK are usually among the second half of the countries to receive, then unbranded with Sony are among the last of them (which I think is kind of weird, Coming from Samsung, unbranded got theirs first which makes sense, no bloat ware and no operators meddling and tinkering with the software, but hey ho, hopefully won't be too long).