"when I reserve space say in DSEG, ASM51 is quite happy to let me use the data directive (for example) to assign a symbol to one of the addresses I thought I'd reserved. I was expecting some kind of warning? "

Its simple - for one data memory location you can have lables or names like say for example
loaction 45h can be assigned name VAR1 and TEMP1 by using differ directives.

when you use DS assembler automatically assign address to you variable as you declare them , you don't have to remeber address but if asign a symbol to address you yourself have to remeber which location I have assigned to which variable