I have googled my suggestion of an A to B charger, and it seems the Sterling one has an AGM voltage setting, but costs more than £200, so it seems not to be cost-effective. To reduce the charging voltage you need to send a higher voltage to the regulator, but I have no idea how that could be accomplished without considerable electronic design experience to make a DIY board to put between the field/aux diodes output and the regulator. That would fool the regulator into thinking the charging voltage was too high and cause it reduce it, but you are only talking abut a few 10ths of a volt. A failed (open circuit) field diode can give a too high output voltage because it causes a slightly lower sensing voltage to the regulator, so did the chap at the specialist physically test each of the three field diodes? To be honest, as someone else has already suggested, I think that you may well have one or more shorting cells, it is shorting cells that usually produce the nasty gas, but that may be because the short makes them hot so they drive out. They might have died out because of the high solar voltage.
