MarkT: Can you post a link to the datasheet or info on this module? Can you explain exactly how you are testing it?
I have attached a photo and what I think is the schematic for this version of the board. That's all I have. To test it, all I did was to insert the power adapter plug into the black socket on the board. I did not connect anything to the supplies, and in fact jumpered both sources to an off position. The 5 v regulator heats to untouchable in 2-3 seconds.
So what "power adapter plug" - what polarity, what voltage? - and is there a manual / datasheet for this module? Is the polarity of the barrel jack socket marked / indicated (believe it or not these things are not standardised).
It does sound like a short circuit at the 5V side of things - do you have a multimeter?