Yes, that was it!

The provided assembler does not match Keil's naming conventions, so it does not link with the objects built by Keil's C51.

What I did was:

1. Create a "dummy" file, modulator.c, with "skeletons" for the required objects:

/*
* Andy Neil, Antronics Ltd.
* Modulator.C
* Comments: Keil C51 definitions corresponding to the definitions in
*           the Modulator.asm file from Philips in support of their
*           Application Note AN10210, "Using the Philips 87LPC76x 
*           microcontroller as a remote control transmitter"
*/

// Data "objects"
unsigned char    system;
unsigned char    command;


// Code "objects"
void RC5_Init(void)
{
   // No code here - just want to get interface, names, etc right!
}

void Send_RC5(void)
{
   // No code here - just want to get interface, names, etc right!
}

2. Build this using the SRC directive to create a Keil A51 Assembler source file, modulator.src.
The generated modulator.src shows the necessary names etc to link with the Keil C51 object files; they are:

?DT?MODULATOR        SEGMENT DATA   
         PUBLIC command
         PUBLIC system
         RSEG   ?DT?MODULATOR
         system:   DS   1
         command:   DS   1

?PR?RC5_Init?MODULATOR                   SEGMENT CODE 
	PUBLIC	RC5_Init
	RSEG  ?PR?RC5_Init?MODULATOR
RC5_Init:
         ;Body of RC5_Init follows unchanged

?PR?Send_RC5?MODULATOR                   SEGMENT CODE 
	PUBLIC	Send_RC5
	RSEG  ?PR?Send_RC5?MODULATOR
Send_RC5:
         ;Body of Send_RC5 follows, 
         ;with references to _system and _command changed.

3. Modify the supplied modulator.asm as shown above.

Now it all builds!