Well, I've fixed it- Seems my CS line was not being held low hard enough! The SanDisk devices were causing some slight ringing on the CS line when data was being output- enough to de-select and re-select the device a couple of times, and with the read line held low throughout, pumping out a byte of data for each 'bounce'. Buffering the CS line resolves the problem.

That's kept me busy for over a week now...

Thanks for your help.

David