poor mash efficiency

[bigdave]

On my last brewday I only achieved a mash efficiency of around 67% but I'm not sure what the most likely cause is? I don't treat the water (apart from 1/2 a campden tablet), my mash tun seams to have developed a leak around the tap (and seems to leak heat too) and I'm not convinced my £3 glass thermometer is all that accurate (see other threads about my crappy thermometers). Is one or all of these likely to be the culprit?

[jonnyt]

67% is above the 65% I get with BIAB on 15 plus brews so in perspective it's not bad at all.