Repressurizing boiler- can I do it?

[50plusabit]

Hi I have just turned on my central heating for the first time, in a new house. When I moved in plumber came and checked it all and it was ok. I turned it on today, and the rads were hot. Came home this afternoon and rads cold, the light on boiler says pressure, and pressure dial has dropped below 0, and supposed to be between 1/2.
Now bearing in mind I am a woman who is not techy minded, is it an easy thing to do?
Its a Heat Line S24/S30 , Im told its not a great boiler, but I inherited it.

 

[Bahco]

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