From Scratchpad

For technical information on the Z80, three very good resources are:

• Thomas Scherrer's z80.info: official Zilog manuals and the like
• Sean Young's The Undocumented Z80 Documented: undocumented features
• memptr_eng.txt: the undocumented flags after BIT n,(HL)

[edit] Additional information

Mark Woodmass has confirmed that bits 3 and 5 of the flags are copied on all BIT instructions on both a Zilog Z80 and an NEC clone (stated on the link above as being "unverified").

Stuart Brady has discovered that how bits 3 and 5 of the flags are set after the SCF and CCF instructions actually depends on the variant of Z80 in use. On a Zilog Z80, bits 3 and 5 of the flags were set by ORing their previous values with that of A. On an NEC clone, bit 3 was unaffected, while bit 5 appears to be set by ANDing the previous value with something unknown. Previously, Ian Greenway has tested this and discovered that bits 3 and 5 were simply copied from A, although it is not known which CPU these tests were run on. For more details, see here, here and a more recent investigation here. Any further information on this is welcomed.

[edit] Replacement Z80s

If you're looking for a replacement Z80 for a duff one currently in a Spectrum, these aren't hard to source as the Z80 is still manufactured by Zilog. In the UK, one source are Farnell, where you want their part number 1081890.