The adverts are not "completely different" - the Feltham job is a relief job at a large signalling centre where you would cover turns at that location that need to be covered due to staff not being available for any number of reasons (this attracts a 4% allowance on top of the basic salary). A relief signaller whose home station is Haslemere would also cover turns where staff are not available, but the relief staff at Haslemere would also cover a couple of other signal boxes either side of that location, on any given day you could be covering a turn at one of several (single manned) boxes.
The higher a relief payment (in the Haslemere case 18%) the greater the distance in mile terms from your home location you will cover.
You ask "how long" a relief job is? As said before a relief job is a full time permanent job (unless advertised as a fixed term contract). You will be appointed as a relief signaller. If you wanted to move to a different box/location/non-relief job you would need to wait for the vacancy to come up and apply for said job.
I appreciate this is a long thread, however, I personally find that if you do a search "on this thread only" in the search box, many issues have already been answered re locations, shifts and various other points which will help with your research. You could try searches for "Feltham", "Haselmere", "relief" which will all bring up information (for example, doing that I found out what boxes a Haslemere relief man covers).