Problem located & sorted.
Using generdawg's photo:
regulator valve bottle selector.jpg
The small gas leak was escaping at the rear of where the brass connection screws into the bottom of the regulator... to accept the copper pipe connector.
Strange that we didn't notice it when the soapy water was used. Fortunately the mate - BobK - dropped in yesterday & noticed it by looking between the front of the van & the back of the gas bottles, using a dark green bush as a backdrop... the slight gas venting was then 'visible' !
A quick nip up with the spanner - less than 1mm - sorted that.
Phew... thanks Bob.
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