Boiler making moaning noises, also no pressure in system

I have a `Chaffoteaux & Maury Britony Combi 80` combination boiler. Not sure how old it is (at least 5 years old as it came with the house and I have been here 4 and a half years). It has had a lot of problems with losing pressure over the years so I have called a heating engineer in on a number of occasions, then just over a year ago it lost hot water completely and had to have a new diverter valve (sorry, not totally sure of the terminology). The engineer also serviced it at the same time. In the summer of this year I lost hot water again and prior to this it had also been making a moaning noise from time to time (the closest comparison I can think of is the Wookie in `Star Wars`) so I had another engineer in to sort it - I`m not sure what he did but he didn`t have to replace any parts and everything was fine afterwards.
All was fine for a few months, then recently the moaning started again - occasionally at first, but now it happens whenever I run the hot water. There is also no pressure at all according to the gauge. I have tried adjusting the three valves sticking out underneath, but really don`t know what I`m doing and one of them made the boiler start banging so I decided to leave it alone! I`m reluctant to call anyone out again as it`s fine for a while then starts again, but the house is on the market so I don`t want it to happen for the new owners (not for a few months anyway...). Any help would be greatly appreciated,
Jason

first you need to get the pressure up look here

the boiler is prob moaning cos its empty

Making a lot of guesses here but I dont think that you know how to repressurise it! You could look at the FAQ on this site.

Probably whoever you called rather than showing you how to do it just repressurised it and charged you.

Alternatively you may just be one of those people who just refuse to learn how to do it themselves and prefer to pay someone to do everything for them.

Tony

I didn't mention it in the first post because it didn't seem relevant, but I've had 2 engineers out - the first showed me how to bring the pressure up by turning the valve on the filling loop, the second disconnected the filling loop and told me I didn't need it but didn't tell me to reconnect it if the pressure dropped again - so I followed the instructions in the link in kevplumb's response and now have pressure in the system and can do the washing up or shower without a chorus of Wookie moans!
And yes, normally I would prefer to pay for someone who knows what they're doing for stuff like this - it's not that I refuse to learn how to do it myself (if that was the case I'd have just gone ahead and got someone in rather than posting on here), but that I'm concerned that if I do something wrong it'll make matters much worse and cost a lot more in the long run. However, sorting this little problem has given me much more confidence to sort similar issues out in future. Thanks for your help!

I did exactly what kevplumb said and i worked great. My central heating is running a dream now. Thanks kevplumb