You can have more than one 15A plug on a circuit but it all comes down to how it is wired... No rules against it these days. Obviously most 15A points are dedicated (require the full amperage), so are usually single point ccts. More than one, larger cable and higher rated protection device.

I wouldn't recommend putting one on an existing cct. A lot of people just put one below the switchboard if it is in the carport.

As a side note, please don't file the earth pin!!

If you are going to dodge it up use the short lead adaptor that most people use. Just use large gauge flex cable, like 2.5mm sq. Also check wiring on packet of plugs. Filing the earth pin will also void any warranty.

NB: Standard double powerpoints are rated at 10A only. More than this WILL do damage...

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Carl
Previous Owner & Driver of 1 Morris "It's not a Cooper!!" 850

I believe it is basically open slather on power points these days as to numbers on a circuit, we hinge more on the capability of the protective circuits to do their job more than we used to.
Prefferably, I would like 6mm from the box to a dedicated high load power point, particularly if you are going to use it specifically for welding.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.