Trouble understanding rated values for a transformer
My question is therefore does full load mean that the primary voltage
is at the rated value or the secondary? i.e that V1 is 500V but V2 is
not 1000V?

On full load, V1 is 500 volts and V2 is 1000 volts but, on no load, V2 will be somewhat higher than 1000 volts depending on how good your transformer regulation is. In other words, the turns ratio is chosen to counteract the droop in output voltage when taking full load current and, this inevitably means that no-load output voltage is greater than the rated voltage.