Help please for a rookie!!

I have a sealed pressurized heating system running off an oil-fired
burner. For some time now there has been a loss of pressure in the hot
water taps, to the point now where if one tap is on nothing will come
out of any other tap. The cold water is fine. When it was first
installed five years ago the hot water was the same pressure as the
cold.
I remember vaguely having a problem with a leak coming from the top of
my hot tank a couple of years ago from a little red knob which was
loose. I tightened this down, which i'm not sure I should have as it
looks like some sort of pressure relief blow-off.
Any help would be greatly appreciated