I used to run a railway station and 'permitted routes' are a nightmare to work out. There's a website specifically devoted to the subject, where you have to get to grips with 7 pages of diagrams and detailed instructions before you can even start to find out what is, and isn't, a 'permitted route:
http://data.atoc.org/routeing-guide
However, a quick look at the train route for the Coventry to Macclesfield journey shows that you'd normally be expected to travel via Birmingham, Wolverhampton, Stafford and Stoke-on-Trent. Birmingham to Chelsford is via Stafford, Crewe and Wilmslow. So the only obvious overlap is on the Birmingham to Stafford section.
So you'll need to buy an additional ticket. The quickest way would be to change at Stafford for an hour's journey to Chelford for £17.90 (single). The cheapest way would be to travel through to Macclesfield, for an hour's journey to Chelford, costing £7.80.