how to regulate gas?

I agree

The "gas man" is either completely incompetent & ignorant of how modern boilers with a premixed burner operates with a modulating gas valve on a zero pressure governor

Or he is a rogue trader out go fleece the gullible

If in doubt about anything regarding the workings of the boiler ALWAYS RING THE MANUFACTURER FOR ADVICE

They will tell you everything you need to know IN SIMPLE ENGLISH

Q
September 2015

He just wants more money from you - get shot of him.

Why?

Your Central Heating is Good
Your Hot Water is Good
There are no Error Codes
Clearly the circuit board (PCB) is working

He is a charlatan - a swindler out to rob you.
All boiler engineers know full-well that the control board just opens and closes the gas valves - they do not regulate the flow other than on/off.

You do not actually say what your problem is - all you mention is the problem the boiler engineer claims.

Dismiss him from your service.

Good Luck...
September 2015