It should be about 0.7 when cold; at 1.5 cold, it could well go to 2.2 or higher when hot, and that's not good for it.
The Worcester website definitely says 1.0-1.5 when cold. The guage drops slightly when the heating comes on then slowly rises to just over 1.5 (i think so don't quote me as I haven't really looked to closely)
Cheers