Remember if the water is in an open container it can't get it hotter than 100°C.
Anyway, Pos is spot on with the stats, you really need to get a gen LR stat, I have found others to be rubbish. I had 2 brand new stats different brands, also looked different so I believe they were probably different makes too that opened very late.
Another thing to consider is where you mounted the jiggle pin, it should be at the 12:00 position, so when the air hits it, it cna pass through and allow hot water to submerge the wax capsule. Also, I always pull the ball or pin out, this means there is always a flow of water past the wax capsule which keeps the temp up, it also saves the radiator from flash freezing in cold weather driving.