ThruUlike - I agree with much of what you say. Unfortunately many of the major factors contributing to the flattening of low-to middle-income salaries is beyond the control of any government, left or right, as it's caused by globalisation. Effectively more and more of our jobs are competing not just with others in this country but with others across the whole world.
Graphic design and software development are good examples: not so long ago, you could make good money as a freelance graphic designer, but these days people can use Elance to hire someone in India or Bulgaria to make a logo for £20, so there goes your day rate.
To make matters worse, more and more of the work that used to be done by clerical employees is being done by computers: for example the clerk in a legal firm who used to archive, manage and retrieve physical client folders has in many cases been replaced or mostly replaced by a search algorithm.
Then there's greater global mobility, which enables bright and talented people to move to wherever the employment opportunities look most promising, which leads to greater salary competition.
No government, whether left or right-wing, can do much about any of these factors. The big scandal in my view is that neither right nor left is being honest with the electorate about what's actually going on, and how much power they actually don't have to affect any of this.