According to the Times report back in July of Des Browne's announcement of reinforcements for the British troops in Afghanistan:
The extra troops numbers will involve a call-up of reservists. There are currently150 serving in Helmand, mostly from the Territorial Army, and 450 more will receive notices to deploy.
Which, given that the total new assignment was 900 troops, implies that half the fresh soldiers will be part timers.
Here's a human interest piece about the imminent deployment of lads from the TA in Gloucestershire to Helmand.
It all seems rather ominous a drum beat to me.