Older boards and availability/compatibility of devices

I know, I know, its been a subject of many threads over the years...but I am bringing it up again.  More a moan I think over the inflexibility (perhaps with good reason I accept sometimes) at times.


Old Volex board - not even split load...all MCB.   In good order, all aspects look lovely.   Ideally, time for a board change....well if you think so ;-)


Now then,  some minor works (circuit extension) would dictate  the requirement for RCD protection.   Solutions, shift the circuit to small outboard BS61008 enclosure; new board...or source and fit RCBO, which seems perfect and is the most cost-effective and simplest...other than...it seems that using another manufacturer RCBO is seriously frowned on... by Volex at least (and I am sure others).   Well I've known this for a while, but never faced a situation where doing so would be the most feasible option.


Is it really that bad to fit another brand device into an old board...what really are the *real* safety risks if the thing is secure and fits.  I cant really think of any other than fluff - assuming its same rating etc and sits nice.


I've heard the phrase type tested and I take it that means that everything in a consumer unit was tested to perform to standards etc when it was made up.  Then putting in a different RCBO means that is now 'broken' as such.


My question and I am just trying to understand the technical and regulatory issues here:  is it not possible at all, to issue a MEIWC to current Regs as a result of putting in a different branded RCBO (I cannot re-do the type testing etc of course!) and where might/is that prohibition backed up in the 'frustrating' Regs Book please ?