My copy of the GMH is currently halfway across the world, en-route to Singapore (it should be going to Canada, but its a long story). However, I distinctly recall writing the following, which should be present in the Combat chapter of the GMH... Although it refers to NPCs, it can be equally used for PCs too.
General Hit Points
First, reserve locational hit points for major villains and NPCs. For all other NPCs, use General Hit Points instead. General Hit Points are calculated on the average of the CON and SIZ Characteristics, rather than on their sum.
• Every NPC has a ‘Critical Wound’ rating equal to half the General Hit Points (rounded up).
• If General Hit Points are reduced to a negative amount equal to the Critical Wound rating or more death is instantaneous.
• Weapon damage is removed directly from the Hit Point total. When Hit Points reach zero, the NPC must make a Resilience test to remain conscious. If the test is successful a Resilience test must be made every combat round until Hit Points are brought up to 1 or more. If the NPC loses consciousness, death results within a number of Combat Actions equal to the Critical Wound value, unless Hit Points are brought up to 1 HP or more.
• If the NPC suffers a single wound that exceeds his Critical Wound rating in one blow, he must make a roll on the Critical Wound table below to determine what additional effects are suffered. The armour value being worn is subtracted from the D20 roll.
• If the blow inflicted is a Critical Hit, add the damage to the D20 roll.
• In addition to the effects detailed in the Critical Wound table, the target always receives an impressive-looking scar as a permanent reminder of the experience.
There then follows the Critical Wound table, which I won't reproduce here.
Perhaps, Gran, as I don't have a hard copy of the GMH available, you could double check your combat chapter to make sure the above is there and, if so, give the page ref. If it isn't, and was cut without my knowledge, then I'll happily post the full rules and crit wound table here.