After doing some googling, I came to the conclusion that the correct torque wrench setting for the crank bolts (the ones that hold the crank arms to the bottom bracket spindle) was 40 Nm. Since tightening mine to that figure, I've had no more problems with them coming loose.the left allen crank bolt tends to get loosened and causes an audible click when the left crank is at 9 o'clock because of the torsion through the BB.
If it's the case, instead of tightening it further, replacing the allen crank bolts with M8 crank bolts will sort it.
http://www.ebay.co.uk/itm/Pair-of-Crank-Bolts-for-Square-Taper-Bottom-Bracket-M8-/262344323019