Over the past decade a bitter battle has been waged to shine a light into an industry that manages £6tn of savings including vast pension wealth.
It is a fight to bring the underlying costs and charges of investing out of the shadows.
Although individuals and pension funds have always seen a figure for explicit costs of fund management, they were unable to see other costs which can seriously erode the value of their savings.
Transparency campaigners say that for years the UK's investment management industry used its substantial lobbying power to resist efforts to expose these hidden costs.
Evidence of the impact of such costs came first from Railpen, the pension fund for railways workers with 350,000 members. It had to hire a forensic accountant to dig deep for the costs that never appeared on the invoices sent by investment managers.
Railpen's director for trustee accounting Victoria Bell says the costs included commissions, brokerage fees, stamp duty, due diligence fees and legal fees. Pension fund accounts for 2011 showed total costs of £90m, but the forensic search uncovered an extra £200m, some of it from layers of fees charged by funds investing in other funds.
Railpen was then better able to judge value for money, looking at true costs, as well as returns. Negotiating down some charges and replacing some fund managers helped it cut costs by £70m.
Smaller pension funds do not have resources to dig down like Railpen. Actuary Hilary Salt is a pension trustee who has found it hard to get at hidden charges.
At our request, she crunched numbers to work out the long-term effect of 2% hidden charges and 1% explicit charges on a £100m pension fund, over the course of 30 years.
Shockingly, the value of the fund would be reduced by half. Hilary Salt finds this "infuriating", and wonders how many closed pension schemes might have stayed open if hidden charges had not drained away value.
It's not just pension funds affected, but also retail investors - individuals who put money in share ISAs or self-invested pensions.
Gina Miller, who's better known for her court actions over Brexit, runs wealth management firm SCM with her husband Alan. She says her firm reveals all the costs of investing and others should do the same.
As an example she suggests explicit pension charges might amount to 1% of the money invested, but the hidden charges could add another 2%.
She says: "If you paid £100 for something, got your statement and saw you'd paid £300 you'd be cross. That's the sort of thing the industry has been doing for a very long time."
Gina Miller told me campaigners like her have been sidelined and attacked. Her nickname among asset managers, she says, is "the black widow spider, because I am bringing down the industry all by myself".
Another industry insider turned campaigner is Andy Agathangelou, who in 2015 launched the Transparency Task Force. He says he has witnessed numerous things that led him to believe the industry is "pre-disposed to misbehave if it's given the chance".
Three years on, the TTF has 300 members and a presence in 18 countries. Its teams meet with industry bodies and regulators, and make proposals for transparency across financial services.
Its founder acknowledges some fund managers are transparent and "consumer-centric", but says others are not. He argues profit margins show "there is a status quo many of them would like to keep".
Average profit margins were 36% between 2010 and 2015, according to a ground-breaking study into asset management by the Financial Conduct Authority, which raised concerns about weak competition.
The UK asset management industry is represented by the Investment Association, which in 2016 published a press release dismissing the idea of hidden fees and charges as "the Loch Ness Monster of investments". In other words, it was only a myth.
When I asked the IA's current chief executive Chris Cummings about the Loch Ness episode, he admitted the "language was regrettable" and said the organisation was "entirely committed to making things clear to customers".
The IA is now working hand-in-hand with the regulator the Financial Conduct Authority (FCA) and others, to hammer out voluntary guidelines to reveal all the costs of company pension investments. Significantly, the man chosen to chair the group writing the guidelines is Dr Chris Sier, a well-known campaigner for transparency. The FCA says progress is being made.
But it could have been made much faster. In Amsterdam I met Eric Veldpaus, whose nickname is "Mr Cost" because he was a pioneer of the increasingly global transparency movement.
In 2011, Dutch pension funds had been hard-hit by the financial crisis. In response, Eric Veldpaus designed the first guidelines so that fund managers would have to show all costs to the pension funds that employed them, including the trickiest of all, transaction costs for buying and selling assets.
It was voluntary, but it worked because Eric Veldpaus knew everyone in the small Dutch pension market. He got a consensus from pension funds, government, regulators, and most importantly the asset managers themselves.
He says: "They knew what I wanted and said, let's do this."
Within two years, pension funds questioned the high charges they could now see, and negotiated them down. Sharper competition brought down the real cost of investing and also profit margins for asset managers.
That's the prize being fought for in the UK.
Perhaps the biggest step forward so far is European legislation known as MIFID II which came into force in January 2018.
It compels asset managers to reveal underlying costs to clients, though it does not cover pension products. Early results, analysed by the Lang Cat consultancy, suggest investors in some top-selling funds are paying 30% more in costs than previously disclosed.
Has the battle been won? I've been reporting on the financial industry for nearly 30 years, and I suspect there may be more skirmishes to come.
This story will be explored on In Business on BBC Radio 4 at 20:30 on Thursday, 25 January. It will also be available via the In Business podcast page..