It doesn't say on any Sentinel X800 bottles that I'm holding right now that it can be left in the system for a week. It says under application on the label on the rear, the following:
To clean in accordance with BS7593 Sentinel X800 should be circulated with all valves open for 1 hour. After cleaning, the system should be drained and flushed through all drain points. If required, can be dosed up to 24 hours in advance....nothing mentioned anywhere about a week.
On the other point, allowing sludge, metal, rust and all the crap that X800 has dislodged to circulate in a 25 year old microbore system for a week will block manifolds, old valves, block dips in microbore pipework and make the system worse before it was flushed. Pointless.