Topped her up, ran her with caps off, no bubbles, caps on, cool down, coolant at same level. Same this morning, even at operating temp and expansion cap off, no bubbles, squeezed top hose, no bubbles. Ran her for 20 miles, temp never moved, heater on or off, stayed at mid point. Maybe, just maybe, the problem's been solved, for now. But fear not brethren, it's a RRC, so I'll be back! Thanks for all the help.