I finally managed to get a stable framework now (more or less), complete with interrupts. I did have to go looking for some answers though as I was getting nowhere fast! Turns out when the 65816 is in native (16bit) mode, its IRQ vectors move to $FFEE and $FFEA. Oh well, live and learn (again). When I did this on the SNES I would have done that the 1st couple of days then forgotten about it!
Anyway, now that its running the real problems with block transfer routines show themselves; you can't interrupt them! Yup, on most CPU's the next interrupt happens at the end of the last instruction - EVEN if that instruction takes seconds to complete! So, looks like I'd need a loop around a few of these, or not use it at all!
I guess for the final BLIT, as long as I knew it wasn't going to interfere I could use it, but it does look like the best/safest way is simply to use lda/sta instructions, Oh well.