boiler pressure drops to nil after use!!!?

i can use the hot water all day long and pressure stays same... i put central heating on and it works perfect, pressure rises slightly which i believe is normal but when i turn heating off the pressure drops to nil within half hour and i have to put pressure back into it to make it work... what does this mean?
I know i will need to get someone out but would like a rough idea of why its doing it and how much its going to cost?

christine
October 2012