Glow-worm K.F.B... how to fill??

my brothers central heating sounds like a kettle without water when he turns it on, the pump in the upstairs airing cupboard sounds like its dragging air through and the boiler bangs also. when the system is off and i attempt to fill up there is no pressure... i know HOW to fix this but there is no pressure gauge, filling loop or ANY cold mains feed ANYWHERE!!!! i know i cud try a back fill but can someone tell me how the hell these things fill up?? i am an experienced plumber but im fairly young and i believe this thing is 32 years old!?!?!?