Apologies for yet another post about a central heating problem.

My house's mid-point valve (an ACL 779H340-30L0) hasn't been switching
properly for a while. I've disassembled the actuator, and there's extensive
arcing damage to the microswitches, PCB and actuator casing, so I think it's
time for a new one.

It seems that spare actuators for the ACL 779H340-30L0 are no longer
available, so I've ordered one of these, which will require a partial drain
down of the system:
http://www.heatingcontrolsonline.co....lve-p-256.html

I have a couple of questions:
- There's nothing in the loft to indicate which (if any) inhibitor is
currently in the system. Would something like Sentinel X100 be compatible
with whatever is already in there when refilling?
- Does this sound like a plan? Does anyone know of a source of new actuators
for the ACL 779H340-30L0 so that I don't have to drain down the system?

Thanks