I do apologise if this has been covered elsewhere, I've found numerous, related posts, but have yet to find a clear, definitive answer on this particular.

I've heard that it's possible to use certain mechadendrites to make an attack as a reaction, though I cant find any rules referring to this. I'd greatly appreciate a page reference (I have access to the Rogue Trader/ Dark Heresy core books) and/or an explanation of the particulars.

I'd especially be interested to know what the consensus is on using the servo arm in this manner in melee, or on mounting ranged weapons upon it for ranged use.

Thanks for the patience.

As far as the Servo Arm it's pretty simply stated in the main description where it gives the rules for using it as a melee weapon. "The Techmarine may strike with the arm as his Reaction for the turn, or as a Standard Attack durring his turn (so long as the servo-arm only makes one attack per round.) The Servo Harness entry talks about options with the weapons on it.

The Ballistic Mechadendrite has it's rules in it's entry in Dark Heresy.