To be careful i bought two types of heat sink and a fan, the fan was part of the 'clear case' package.
The larger of the heat sinks is called Armour and not only encloses the board but conducts heat from the three main chips, however neither of the cases, which are dimensionally the same can be fitted around the Armour, so decided to use the small heat sink and the fan.
It is debatable if the fan alone would be more efficient, but some advantages of the heat sink ard a) if the fan fails, and it is quiet so failure may go unnoticed, there is still heat dissipation and b) the heat sink continues to function when the power is switched off. I have also incorporated a CPU temperature monitor on the desktop to keep an eye on how the CPU is faring.
Sadly the heat sink was too tight a fit to use but The Pi Hut sent another, though i didn't ask for one, i was just going to file the original one down to fit.