Inheritance was made in 1967, so predated the Python Spanish Inquistion sketches, in which trouble at t'mill featured, by three years. However, neither was likely to be the origin of the phrase - it's a north country expression of much longer standing. 'Trouble at t'pit' is also used. Mills and pits were both troublesome, as sites of industrial accidents, diabolical working conditions and labour disputes - I think the expression reflects sardonic northern humour at its best. There was always 'trouble at t'mill'.