what causes a combi boiler to get very hot and smelly?

Hello,
Can anyone help? I have a 10 year old boiler that was in good working order, no bad odours, smells or noises. Recently have added two new radiators to the central heating system, and also replaced another five radiators with supposedly more heat efficient double convector radiators.
Immediately after the engineer finished the radiators didn't get hot, then after approx an hour or so, they began to get hot, the boiler became very hot and the pressure gauge dropped right down to zero.
The boiler has never been right since, this is approx 2 week's ago. Now after being switched on for less than an hour it becomes really sickly smelling. When it is turned off, it starts to click & make a funny noise, as though it is cooling down?

Is the boiler totally knackered? would appreciate any advice-help anyone can give.