Intermittent No Wireless Signal

[#1]

Several times over 6 months my computer cannot connect wirelessly although wired is OK. There have been no changes to settings. This also happens to a friend with Plusnet on the same exchange.
Different laptops, different routers make no difference and the internal pages of routers indicate  wireless OK but laptops indicate no or very weak  signal.
How can  this be?

[#2]

A wireless problem is a problem between your computer and your router - that's as far as the signal goes.  ISP and exchange make no difference except that if you're with the same ISP you might have ended up with the same model of router - but you say not.  So a wireless connection problem could be that your computer is too far from the router, or there could be interference or it could be that the wireless equipment in your router/or computer is playing up. 

[#3]

My wireless signal strength in the study drops off every now and again. It appears to coincide with the presence of another relatively strong signal from a neighbour's network ie interference.
Try searching for other networks - if they are using the same channel as you, try changing channels.

[#4]

As brewerdave says, you need to change wireless channels. All sorts of electrical equipment kicks out interference in the band that your router uses so you might not see another network listed. You've got 11 channels so work your way through them to find a clear one.