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It's a task that barely crosses most Londoners' minds. But it's crucial to the functioning of this and every other big city in the industrialised world. Well before 5:00 each morning, Guadalupe Noristz and thousands of other cleaners, most of them immigrants, get up and make their way to the capital's offices, shops, and hotels to make them ready for the new day. The question is why working life for Guadalupe and others undertaking this vital task is so precarious.

Although UK workers theoretically have strong protections, many of the 700,000 people employed as cleaners describe a code of long hours, low pay, and the constant fear of arbitrary dismissal. The problem is particularly acute for those like Guadalupe who have worked for outsourced cleaning contractors. Guadalupe, who's originally from Ecuador but now a Spanish citizen, says she lives in fear of losing her job.

Guadalupe's frustrations include the short hours per day she works each job.

Lucila Granada of the Latin American Women's Rights Service, a group that works with the women, says there are too many violations of the employment rights of cleaners, ranging from underpayment to harassment and even violence. The problems extend to similar roles in hospitality and catering.

There's too many violations of employment rights in these sectors. And perhaps the most common one is lack of payment or underpayment of wages. So we have a lot of women who come to us saying, I didn't get paid, and then I was dismissed, and they owe me two, three weeks. Or at my job, this is an ongoing issue... I'm not paid for say, extra hours. Or they always deduct a few hours every week.

Dalia Quinonez Guerrero, originally from Peru but now an Italian citizen, tells a typical story of working for a company that supplied housekeepers to hotels. She says she was paid a fixed rate per room. She'd have had to clean three rooms an hour to earn a national minimum wage, but two proved the maximum possible.

Although cleaning contractors mostly insist they try hard to be fair, Guadalupe's and Dalia's experience is a common one among the thousands of foreign women working in the sector. That reflects their very vulnerable positions, according to the man who sets strategy for labour enforcement in the UK.

There are a lot of cleaners that are supplied by agencies. Some, of course, will work in lots of different workplaces. The workplaces are themselves oftentimes small, not always, but oftentimes, small. And many of the cleaners, probably including the ones that you've spoken to, are immigrants and potentially vulnerable. They won't know their rights, and they probably don't know how to complain.

Part of the answer, according to Lucila Granada, is for the workers who benefit from cleaners work to be aware of their predicament.

I think that the people that are working in these very nice buildings in the city and in these spaces that are really - you look at them, and they're immaculate. They have really good HR processes and great benefits. They don't realise that often in those same spaces, there's a different parallel workforce that is being exploited - the outsourced workers who will not benefit from those policies, and they are being exploited. I think that people don't know that this is that common, because it happens overnight - because it happens when they're not around.

So David sees it as vital that enforcement is taken more seriously in the UK labour market, which is far more flexible than in continental European rivals.

You can only make sure that the flexible labour market works properly if you do enforce the minimum standards. The flexible labour market may mean the vulnerable workers get very badly treated. Well, they shouldn't be. And we've got to make sure that those people are properly protected. And we have the resources, the penalties, the compliance to ensure that they get their just rewards.

For the moment, however, most people involved believe labour inspectors have far too few resources to tackle the problem effectively. That leaves the women themselves as some of the most powerful voices for change. An increasing number are organising and protesting. They hope that might finally mean London office workers can sit at their desks, knowing that those who clean them did so on reasonable terms.