If the TV in question has the RFI ferrite's installed on the power cord to the TV then that would eliminate the chance it's directly coming into it via the AC mains.
This is looking to be a EMP pulse from the boilers ignition circuit, they make ferrite snap on's for the coax cable, I would try some on both the satellite and TV coax, this may cut down on direct RFI to the TV/Humax, also since the Humax uses an external DC power supply you shold get one for it also.
I would check to see how the case of the boiler is grounded, is it just through the electrical cord/connection, or is there a real external ground?
If the boiler's water supply piping is metal (iron pipe or copper) then it should be/ could be grounded through the water supply piping.
This however could cause a problem if the two grounds are at different locations, if the two grounds are far apart (case ground and electrical ground at the AC mains service/fuse/circuit breaker panel) it could be causing a ground loop problem, this could make things worse.
I have seen problems where the equipment is grounded through the electrical service ground only, and the equipment is located a ways away from the AC main panel, this was a long ground wire, connected only at the main panel, this cause a ground loop problem and the RFI was radiating off the long copper wire going back to the panel, it acted like an antenna.
Many possibility's here, I wish I lived close by , I would have hopped in the truck with some equipment and helped work this out.
Oh, do a google search for a cheap AM radio from someone at your location, and one with a manual tuner, I found some below 15 USD on amazon, the radio will tell you if its an EMP, and EMP covers a very broadband frequency spectrum, if you hear a POP on the radio then it's an RF EMP and that's a hard one to cure.