With regard to timeframes, it might well depend upon how compliant your neighbour will be when given notice to cease the use of the sawmill, and possibly to demolish some of the building, which he almost certainly will be. Robert Fidler was given notice to demolish his house in 2015 but famously hung on for 7 years before finally doing so:
https://www.thesun.co.uk/money/property/19034937/demolished-castle-surrey-farmer-robert-fidler/
I agree with Paigntonian (which is probably a first!) here. Councillors, rather than council officials, can be incredibly helpful. For example, despite me telling them about a change in my circumstances, my local council gave me a 100% Council Tax reduction throughout a full financial year but then, when they realised that they shouldn't have done, billed me for the full year in one, with no option to pay in instalments. When I protested to my local councillor, she got the entire year's Council Tax bill totally written off for me!