There are a few possibilities (I'm not familiar with the R21).
The expansion bottle cap is probably the one that does the pressure release when the pressure gets too high. It would let water out if it was broken and was releasing at too low a pressure, or it would do the same if the pressure was too high (maybe it was over-filled? - normally they are half full so that the air space can take up the expansion of the water as it gets hot).
Those would be the most likely. Otherwise the pressure can get high due to overheating or various other woes, but if you don't keep having to top up the water then the problem is probably in the previous paragraph.
