Private equity advisers could be forgiven for thinking they are playing a game of lucky dip given the way buy-out firms are farming out work these days.

In the mid-market the likes of Alchemy (Macfarlanes), Phoenix (Travers Smith) and 3i remain loyal to individual firms, or, in the case of 3i, a regional network of advisers.

But at the busy top end of the market it is becoming increasingly difficult to predict exactly which firm will find favour with which house at any given time.

The last six months have thrown up numerous examples. Take Apax, which used to use Ashurst and sometimes Clifford Chance (CC) for its biggest deals. Last year it set up a three-firm panel that also included Freshfields Bruckhaus Deringer and Weil Gotshal & Manges. And it was not simply window dressing.

For its biggest deal this year so far – the £1bn acquisition of Travelex – it instructed Freshfields, while it has also sent a steady stream of work to Weil Gotshal.

Meanwhile in Germany, Milbank Tweed Hadley & McCloy's newly-acquired Munich office is the front-runner to nab Apax's eagerly awaited float of Kabel Deutschland.

After all, it was Munich managing partner Peter Nussbaum, who joined the firm from Freshfields last year, who advised on the house's original £2.8bn acquisition when still at the magic circle firm.

CVC, whose expected IPO of Debenhams is a mouth-watering prospect, is another busy house that has been liberally doling out instructions.

A traditional Clifford Chance (CC) client, CVC has become something of a flirt over the last few years instructing firms including Freshfields on a $1.5bn (£830m) acquisition of power business Ruhrgas Industries in June and Linklaters on the £800m sale of Kwik-Fit in July.

Advisers put this new-found generosity down to the fact that separate CVC deal teams have free rein to appoint their favoured firms.

Another major player, Cinven, has also strayed from its love-in with a particular firm. In January it bypassed traditional adviser Ashurst to advise Fresh-fields on its £3bn acquisition of travel company Amadeus.

Both Ashurst and Freshfields will be vying with each other to get the work advising on the IPO of bingo chain Gala, which is expected in the coming six months and could be worth £1.8bn.

And finally there is Permira, once CC's trophy private equity client, but now no longer viewed as being wholly faithful to the firm.

Slaughter and May, Linklaters and SJ Berwin have all benefited from Permira instructions over the last two years. CC will no doubt be keeping a watchful eye on the house's planned float of chemicals company Cognis in an effort to ensure that it does not slip through the firm's fingers.

Of course, this merry-go-round does not mean that new firms are breaking into the top tier in any great way. It is the same names cropping up again and again. But it does suggest an important new trend.

Advisers report that while all the main firms are benefiting from the surge in private equity work, the private equity houses are starting to wise up to the benefits of keeping firms on their toes.

This suggests that more marketing, more client meetings and more long-term concessions on fees will increasingly become the order of the day if the top firms are to continue scooping the major roles.