low pressure in my Sime format 80c?

I have a sime format 80c boiler installed about 1 year ago.

The pressure on the boiler goes low, for no apparent reason.
We've had somebody from the waterboard come out to check our pressure, which is fine.

We've had a plumber out & he does not know what is wrong with the boiler. After the plumber drained the boiler, it was working again..... but only for 2 or 3 days.

Will draining and or flushing the boiler make it work again? Can I do this job myself?

There is a tap below the boiler, with which you can add pressure to the system & sometimes this has helped. This tap is now working sometimes, but sometimes not & the plumber who visited said that it needed fixing, but that it perhaps isn't worth fixing it, as it'll cost £60 and we shouldn't need to use it more than once every six months or so.

Hope somebody can help.