The uRTC-8 universal RTC by Jon Bradbury is a Real Time Clock interface for Amstrad PCW (or others Z80 computers). It only costs 37 pounds fully assembled (add shipping costs).
This interface cant be used on an Amstrad CPC : I/O are using 16 bits addresses instead of 8.
Jon is also the author of another interface still for Amstrad CPC and PCW : UIDE Universal IDE adapter cards for Z-80 computers which let you connect either a CompactFlash card or a Disk on Module (DOM). It exists in 2 models : uIDE-16 (for Amstrad CPC, but can also work with a 8 bit addressing on others Z80 computers) and uIDE-8 for Amstrad PCW (but can't work on Amstrad CPC).