If the noise picked up on your radio coinsides with the drop of broadband then you can be very sure thats the cause of the problem.
If you suspect it's the sewage pump I take it this isn't something that runs through your consumer board (fuse box & mains switch). The final thing to try is slowly turning off then on each fuse on the board, or just the mains supply as a whole if you have an older board: all whilst listening to the radio @ 612 khz (mw/am).
One of the main problems with REIN issues is that you can have one item tucked away somewhere generating the interference signal, yet as it's connected to the mains, all the local mains cable acts like the antenna (and one with a power supply built in so making things worse) so you pick up the same volume of interference around a wider area, and the source can be any where inside. It's like listening to a string instument, the noise comes from all of the string almost equaly, but your trying to locate whats causing the string to hum (the bow or finger plucking it
Good grief, I do like my analogies. sometimes I wonder when I read some back, what are you on ?
Sounds as if this could be your situation. Something (ie the sewage pump) generates the noise, this goes back via the mains a short distance (the supply within your and up to your house).
When using a radio to locate the noise, turn the volume down low when you hear the interference, this make it easyer to hear if it's stronger in any one area
as you go around a suspect area remember to move the radio at 90 degrees horizontaly (and verticaly) just by roughly rotating your hand and wrist a bit. the MW antennea in the radio will pick up signals pointing in one direction better than another (just like when your tuning into a radio station, you sometime need to turn the radio around a bit to get a good reception.
In order to state for certain if the pump is the source then you'll need the interfernce to be on and picked up through the radio, then some one will have to switch the pump off, if the interference goes you've pinpointed the source (sounds like the water company will have to do the switching as it's their equipment which is suspect). Now you've hopefully identified the source you get the next problem...........
As Astral pointed out, the Post Office had the remit to invesgtigate (and power to intervene) with interference issues. Now with a combination of deregulation and the fact we are now swamped with all sorts of elecritical noises and inevitably some stuff may clash with others.
(incidentaly, anyone reading this have a new up to date fancy washing machine which changes programs mid wash ? is you wireless router within a few meters, if so move the router or better still change the channel on the router, I'm being serious this is genuine ! can't mention the make of washer, lets just say think of a thirsty hunting dog
)
BT unsuccsesfully tried to get the laws tightened and clarified over this some time ago in anticipation of just this sort of problem. If a source is suspected form a 3rd party, things are very very hazy as to what can be done about it. I've had such a fault to investigate a little while ago, after several visits where I was ther while the inteference poped up, it seemed to possibly come from one of 2 nearby properties. could'nt do anything about it as I didn't have access (& no none would answer the door)
One last thing, it's very easy to suspect overhead cableing be it telephony or mains to a greater degree than other sources. This is because you can see it. last 3 out of 4 interfernce issues i've come across the noice was transmitted back through the underground cabling. So if you ever see an openreach engineer stooping along the pavement with a radio and another small box which makes noises like Sweep from Sooty and Sweep you know what their up to !