I think that ATC is only required at airfields with more than a certain number of scheduled movements per week. Thus WR will not need a controller, but NP / NR / GS etc do. No controller = no scheduled movements during that period.
Also dredging stuff up from memory, I think Air Nelson always needs ATC because they operate aircraft above a certain capacity. Risk management means they always have to have ATC (CAA rule, not management decision.)
Happy to be corrected as always. Can't be bothered downloading Parts 125 & 121 to check...
O8