If it is the rear chain then it is engine out job!!! You will still need the rear jackshaft bolt as this is TTY (torque to yield ie stretch bolt). You can inspect by removing the cam cover tho there isn't must room to see a lot. You may be able to detect a slack on the chain on the inboard side and or see wear on the casstte top mounting bolt (mirror may help).
For the right price i have a rear cassette BNIB that i'm willing to sell, so all you would need is the jack shaft bolt (IIRC about £5 from Ford UK), the bolt cover, oom12 kit (new manifold o ringsand updated front tensioner) and a new rear tensioner.
You would still need the tool kit to get the cam timing spot on tho there are threads in the US section where by people have managed by making up their own tools, specifically the cam holding tool and the chain tensioning tool.
But for my money if the engine is out for the few extra pounds and a bit more work you could replace all the front guides also. So i would buy the kit and all the other stuff and do all the chain gear. That is unless you have a record of the front stuff being done already