RF is very notorious to track down, without really decent tracking equipment you can believe you have the source then it'll turn out to be bouncing off buildings and will be elsewhere..
Your neighbour probably has an imported 2.4ghz device that exceeds UK power limits ( I have a few devices like that but only use them at events for video transmission over a few hundred meters
) The overpowered and a lot of times unfiltered devices can interfere with other users to a point where the signal of some cheaper routers can be swamped making them totally unusable...
It's not though normally upto VM to track these down and legally they can't do sod all apart from reporting the offender to OFCOM