The IET has said that poor installation by "cowboys"  is often the cause of interference. They need to follow good practice such as using paired red-black cable (like speaker cable, a stuck together pair) when running dc cables and not to form open loops. 
The IET (previously the IEE) has just brought out a series of codes of practice for installation ranging from small domestic systems to large scale PV systems. It's £59 to £69 if you're interested and follows the same style as the wiring regulations.

Discussion in the IET magazine has shown that particular makes of inverter/regulator are known to cause interference.
The majority of installations do not cause problems and I have not had any problem here in Blunsdon Towers from neighbours systems. 

Following on from Den's comment about Swindon BC's initiative to work with one supplier/installer for a discount, once the details are know the club could perhaps vet the track record of "the chosen one" or ask the Council to make it a requirement that the installer uses the IET code of practice?