NCT visual re-test only - booking required?

There is a time limit, it's printed on the sheet that came with the test result, and if you go beyond that date, it's no longer a free visual inspection, it's a complete retest at the full fee. Some of the test stations that operate extended hours won't do a visual during the shift changeover times, or in the last half hour before closing, so if you're planning to use the same station for the check, have a look at the notice board to see if there are any time periods where they won't do visual checks, as it varies from station to station.

Other than that, yes, you can just turn up and ask them to do the check