I certainly would be interested in the results. I have had no reason to doubt the accuracy of any Brymen I have received but I have not had access to such a high accuracy reference.
Accuracy at one time is unfortunately not the key Factor for professional use.
Dave often replied it: its all about the history and the confidence level over time (repeatability) what is needed to build up the trust for a measurement device.
We have a small in house cal lab (not accredited) to calibrate our internal measurement equipment.
Calibration in our case means only the check if the instrument which is defined to be controlled is in cal or not.
For example: comparing the defined values and calibration steps from a fluke 289 against the calibration values of a Fluke 5520A.
The result after checking and storing the Data is simply the answer: cal passed and cal sticker will be renewed.
If the cal process is not asking for, there will be never ever any adjustments done to bring it in cal or adjust it to any direction.
Conclusion: The calibration process only checks if the measurement device (for e.g. Fluke 289) was passed the calibration process without any error after the last check again (verification of the History).
If not, this result can become very difficult, complicated and/or expensive for some Business and you may have to recall all your shipped goods for rework after the risk assessment.
And Yes, you have to do a risk assessment is you are certified according ISO 9000
With this in mind you can imaging that for example a Fluke 87V which is on the market that long and passed in many factories under harsh conditions so many times the calibration process must be a reliable piece of equipment which is hard to beat.
For Hobbyist user above mentioned things don't matter and you can calibrate or adjust your instrument as much as you want and nobody will care about.
it would be very interesting if somebody here in the forum can post some statistics of Brymen multimeters.
For e.g.: we are using 20 BM869S on daily laboratory basis (normal conditions) and our annual calibration cycles where passed since 10 Years without any deviation.