The FTSE 100 has been the bellwether of the nation's fortunes throughout a decade characterised by dynamism, as well as uncertainty. And whether prospering or confronting hard times, the U.K.'s public companies have turned to law firms whose own reputations have long been tied to the country's key share indices.

Using data provided by information company Adviser Rankings, Law.com's Legal Week compares figures from the first quarter of 2010 to the last quarter of 2019 to establish who among the country's top corporate advisers have managed to retain, or indeed gain, the most listed companies on their books over the course of the decade.

The 10-year period has seen some law firms demonstrate unrivaled consistency. Slaughter and May continues to dominate the tables, comfortably topping the FTSE 100 and 250 adviser rankings in the latest quarter. Having added a net of two listed clients in 10 years, the Magic Circle firm now counts a third of the top index's companies as its clients (33), and has added another eight clients to its FTSE 250 roster for a total of 46.

A similar story plays out over in the AIM market, where Pinsent Masons has overcome the Gowling WLG challenge to reaffirm its position at the top of the table, counting 54 AIM-listed companies on its books – just one less than a decade ago.

"We've probably had one of the worst years for AIM IPOs"

Elsewhere, many of the industry's biggest names such as Freshfields Bruckhaus Deringer, Clifford Chance and Addleshaw Goddard have held their ground in the FTSE 100. This quarter, Freshfields retains its top five place – fourth this year, third 10 years ago – while Clifford Chance moves up one to sixth, and Addleshaws holds fast the flag it pitched at seventh in 2010.

Yet the period has also witnessed seismic shifts amid intensifying competition.

Among the Magic Circle, Allen & Overy has expanded its FTSE 100 base by almost 40% since 2010, pushing it up to two places to second, overtaking Freshfields in the process.

DLA Piper and Eversheds Sutherland have also made concerted gains among FTSE 100 clients. The former has risen 20 places to 12th, while the latter's client list expanded threefold from three to nine, jumping six places to join top-table mainstays Addleshaws and Ashurst at seventh.

Across all listed U.K. companies, there have been some meteoric rises. Herbert Smith Freehills, which at the start of 2010 polled fifth with 87 clients on the stock exchange, has seen its listed client base swell by 44%. In doing so, the firm has leapfrogged Slaughter and May to the top for total listed clients, with 125 clients to Slaughters' 121.

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HSF achieved this by overseeing rises in the FTSE 250, Small Cap and Fledgling markets – soaring into the top five in each.

Commenting on the results, HSF co-head of corporate Stephen Wilkinson noted how law firms' culture and diversity has risen up client agendas.

"Clients expect teams from law firms who are servicing them to share the same values and approach that they have in their own organisations," he said, "and that's something we're very keen to talk to our clients about.

"We have some clients who say they want to see just as many women as they do men – some want us to report in on these issues."

Offshore law firm Carey Olsen has also shot up the overall rankings, expanding its client base from 57 to 95 – an increase of two thirds.

On the AIM index, Hill Dickinson has rocketed up the table from 55th in 2010 to fifth this quarter, while Fieldfisher has risen from 12th to third during the same time period.

The reverse has happened elsewhere, according to the data. Eversheds Sutherland and Norton Rose Fulbright, once third and fourth respectively in the overall count, have shrunk to 12th and 10th, shedding a total of 43 clients between them.

Ashurst has also fallen from the top 10 both for overall clients and among FTSE 250 advisers.

According to Pinsent Masons corporate finance partner, Julian Stanier, the market conditions for the coming year might well prove positive after a year of uncertainty.

"We've probably had one of the worst years for AIM IPOs," Stanier said. "We're expecting to see a bounce in that market.

"I've got a number of AIM IPOs on the shelf ready to go, so to speak."

He also believes that private equity "is sitting on a lot of cash at the moment", and that "we're expecting to see an uptick of 'P2P' – public-to-private deals, where they take companies off the market".