Burst discs are like a fuse in an electrical system, they don't normally just go for no reason and just replacing them usually doesn't work as there's a fault in the system and will blow again. Just cleaning the inside of the fill nipple won't fix it as the problem is usually the seal and little o ring at the bottom of the piston inside the reg, if they're damaged or worn or have dirt on them then they don't seal properly thus allowing the pressure to build up inside the reg to the point the 1800psi burst disc pops. Usually only a full reg strip down and service will stop it from going again. Incidentally there's no need to replace the HP burst disc, the HP burst disc is to protect the bottle (stop it from blowing up) should it become over pressurised.
