Bit more frequent than Blue Moons

This cactus was treasured and nurtured by my F.F.i.L. and we inherited it when he passed away (2004) and F.M.i.L. moved in with us.
It infrequently flowers, I think the last time was around 4 years ago. What triggers the flowering I don't know but I don't water it much, not at all during the winter and only a little during the summer.
The flower, which I will blip when it is in full 'bloom' is not very impressive although the stem that carries the flowers is long with small flowers at the end.
Watch this space.