If the water pump has gone then look for a drip of water from under the body of the water pump. If the seal goes inside then there is a small hole in the body to let yo know!
It could be the head gasket, especially after a boiling, look for water in the oil (white mush on the dipstick & in the rocker cover) and oil in the header tank.
Also check that none of the water pipes have a small split and the same with the radiator, also make sure all the jubilee clips on the pipes are tight.
Did it loose any more water after you refilled it? It may have just been very low when you started the trip.