Please ignore my previous question - I should have read the documents first!
The answer is here:
"When using the statistics to monitor current activity, it is important to
realize that the information does not update instantaneously. Each
individual server process transmits new block and row access counts to the
collector just before going idle; so a query or transaction still in
progress does not affect the displayed totals. Also, the collector itself
emits a new report at most once per PGSTAT_STAT_INTERVAL milliseconds (500
unless altered while building the server). So the displayed information lags
behind actual activity. However, current-query information collected by
stats_command_string is always up-to-date."
Regards
Donald Fraser