Some useful replies there - much appreciated.
I particularly want to stick to my philosophy of having a minimum chip count, and serial comms of any kind will need at least one more chip at the Z80 end. If inter-processor comms were going off-board, then I would certainly consider some form of serial.
Although I'll definitely be using a PIC as a peripheral device, ironically I found Eric's suggestion to be the most useful - not in using the Zilog peripheral itself - but rather his suggestion to use a device in a PLCC package.
Without realising it, I'd set myself an upper limit of 40-pin DIL (for obvious ease of proto-typing), but on checking some of the larger quad-packaged PIC's I find that many have 7 or more tri-state I/O ports. With so many ports to play with I could even invoke /BUSRQ and write the I/O data directly into a reserved area of the Z80's RAM, without using so much as one external decoding chip or tristate buffer !
Or I could stay with the PSP, create 2 extra handshaking lines (RTS:- Output Buffer Full, request Z80 read, and CTS:- Input Buffer NOT Full, Z80 may send) to convert a Slave Port into an active port, and transfer I/O data in a more conventional way.
Plenty to think about now.
Many thanks