So the replacement microfilter arrived yesterday so I did the decent thing and tested it before giving them an angrier call. Thankfully this guy was far more helpful and told me almost off the bat after running his tests that the speed we were receiving was ~200Kb/s which is clearly less than the absolute minimum guaranteed speed which is ~500 and still significantly lower than the lowest expected speed of 1Mb (but who is gonna settle for either of those anyway...).
He had no explanation for it though so engineer is coming out on Monday (between 8am and 6pm

).... Will see what he says!
I'll be expecting some serious money off the bill considering I had no internet at all for nearly 2 months and now this.