So, quick update:
After sending and receiving lots of emails to various companies, and doing more research and Youtubing, I think I'm down to probably House of Tone or Cats whisker. Both represent great value for custom made pickups, both made in UK, and both give me a good level of confidence that they can fulfil my needs.
My gut feeling at the moment is to go with Allan at Cats whisker, he's very knowledgeable, very responsive and although he was only initially recommended by a friend and bass builder, some goggling has shown that his pickups are highly regarded by those that have tried them. It turns out that he is (or was) making all of the pickups for Alpher too, so that says a lot!
I'm still a bit undecided on the J pickup, I'd love it to be hum cancelling, but need to accept that it wouldn't give me quite that classic single coil J sound, although how far from that it is in reality remains to be seen. Apparently I could expect it to sound a little less 'open' and dynamic, with some loss of highs and high mids, bit slightly more pronounced lows and low mids.
Still trying to decide wiring also, I keep fancying stacked VT VT a la early 60's jazz bass, but as I've never tried this layout I'm not sure if it makes most sense. If not will probably just go VBT.
Waffle over. π