Many thanks for the replies and suggestions chaps.
The new stick would appear to match the old in terms of spec. Details as follows ;
New DIMM
Integral, Part No. DL1664USNST347XX-Q
Chip markings : hynix 227A HY57V28820HCT-H
PC133 16x64Mb Synch NS 3.3V 7.NS
Old DIMM
Don't know supplier/manufacturer, Part No. 1645-020213-061
Chip markings MT 48LC16M8A2 - 75B
Perhaps the 1664 and 1645 in the part numbers is a clue ? In the meantime I'll consider the one 256Mb stick.
Thanks again.