Interested to know what a 'sonic cleanse' is.

We're not sure what a sonic cleanse is also ... perhaps some sort of ray
gun 8-( ?

I imagine it's an ultrasonic cleaner, which could well help.

I had the primary (i.e. the closed loop) of my DHW (on a Halstead combi)
block last year with the usual black sludge and debris, corrosion products
from radiators because of insufficient inhibition. Replaced the heat
exchanger and put in a Magnaclean (and added inhibitor). Within about six
months it was misbehaving again. The Magnaclean had certainly trapped a fair
amount of black oxide, but the heat exchange primary was blocked again. This
time I tried a chemical clean on the removed heat exchanger using formic
acid (kettle descaler) plus flushing. Not the ideal chemical approach,
hydrochloric acid (brick acid) would be better but this can be bad for
stainless steel heat exchangers. It's been OK for a while; perhaps the
Magnaclean is taking a while to collect the existing circuit debris. In an
ideal world you might flush and re-dose the whole circuit, but that wasn't
practicable for me at the time. There doesn't seem to be a problem with
limescale buildup on the secondary (domestic hot water) side in my system; I
don't think the heat exchanger really runs hot enough.

I'm reasonably happy with the Magnaclean, though. Quite well designed and
easy to clean out (it comes with isolating valves) and it is a convenient
point to add inhibitor, slightly more convenient than via radiators.