Nothing serious, but came back from a drive and parked the Series in the garage, and as things were being put away heard the sound of liquid splashing onto the floor. It was quickly visible that the lower coolant hose on the radiator had failed, with a resulting large green pool on the garage floor.
The lower radiator hose had failed where it connected to the in-line heater, at the point where it stretched over the heater flange. There appeared to have been both blistering and cracking. The hose material itself, away from the damage area, was otherwise in great shape.
I could imagine the hose cracking, as the in-hose heater takes away some of the flexibility between the engine and the radiator, and the installation had been there for six years, but I'm not sure what caused the apparent blistering - wear and tear or a different issue. Perhaps six years of wear and tear + heat + time.
Removing the plug insert revealed that the inner surface of the plug had swollen and split at the sockets. I can't be sure whether it was just the six winters of heating etc. that had done this, but either way the unit clearly had to be removed.
For now, the lower radiator hose has been replaced with a stock unmodified one, and will probably leave it like that. Perhaps next winter I'll use a magnetic-type heater, such as I use occasionally on the underside of the oil pan on really cold days.
Below, the assembly as removed. Had to cut the top hose as it had firmly welded itself to the water pump casting:
The blistering. Or could it be wear from being a vibration point stretched over the heater flange?
The inside of the heater plug:
