Thanks guys.
All makes sense
I have just done some reading up about it and yes, the regulator will get quite hot needing a heatsink, but I don't have room to add that too, so I will first attempt to track down a 12v micro fan.
I have seen them online at 30mm x 30mm x 6mm.
Even a 35mm x 35mm will just fit, so I have two options in 12v.
The one I have now is 40mm x 40mm and it just won't quite squeeze in.
If I have no luck I will return to this 5v fan and have to add a heat sink somewhere.