It seems the beep code, according to the Intel manual, means 'First 64 K memory failure':
Table 36. Beep Codes
Number of Beeps Description
1 Refresh failure
2 Parity cannot be reset
3 First 64 K memory failure
4 Timer not operational
5 Processor failure (Reserved; not used)
6 8042 GateA20 cannot be toggled (memory failure or not present)
7 Exception interrupt error
8 Display memory R/W error
9 (Reserved; not used)
10 CMOS Shutdown register test error
11 Invalid BIOS (such as, POST module not found)
Checking a few solutions on the net they seem to indicate that this 'memory' is located on the motherboard itself, rather than being a ram problem. I can't see that being the case though as the original memory in the PC works fine and boots up every time, surely if it was the case then no memory would work in the motherboard.
I'm still not sure why memory that I believe fits the specification will not work.
I've now tried resetting the CMOS by leaving the battery out overnight and also by resetting the BIOS jumpers to clear the memory, but still the three beeps if the 2 x 512MB are in the Motherboard.
Either the motherboard cannot take the 2 x 512MB OR the ram is bad, although I can't se why both sticks would be bad. I have no way of testing the ram to see if it is OK and no memory to swap into the machine to see if it works, so I'm sort of stuck now!