Inspection in regards to immigration

Today in work we were approached by two people claiming to be checking up on workers rights and immigration.

They asked to speak to some of the employees but after they left we realized they had only spoken to me.

They asked me the following:
name.
date of birth
address
my nationality
my hours
my wage
my wage on a sunday
if i worked bank holidays and how much my wage is for that.
my holidays
i did notice a slot for my PPSN number but they did not ask me that

Myself, my co-worker, my manager and shop owner were all a bit confused after this and im wondering is this normal? does an authority do random checks? and should i be concerned i was the only person asked these details? i was not the only person on the shop floor so it wasnt as if i was the only one around.