Thanks.
Would I need a flyback diode if I used a relay? The one I was thinking about mentions back emf protection which I assumed made it a handy all in one solution, even if it is a crowbar 
I'll try and get to my electronics store sometime and pick up some mosfets (they seem to have the one you recommended:https://www.jaycar.co.nz/2n7000-n-channel-fet/p/ZT2400?pos=1&queryId=098880d04530ee57e5329ade203d3def) , I get the impression I'll be needing them sooner or later.