Thawing haggis safely
Defrosting haggis is no different to defrosting any other perishable food. It shouldn't be at room temperature for more than 2 hours. That means:


Defrosting in the fridge for 24 hours
Defrosting by running it under cold water for no more than 2 hours, then cooking it straight away.


There really is no problem with cooking haggis from frozen (or after partially defrosting), so long as you ensure it's piping hot throughout when you take it out of the oven. Use a probe thermometer if possible to ensure a safe temperature (75°C is a safe bet).