I set up my own personal investment scheme, or SIPP, just over two years ago, but the five stocks I chose have already doubled in value.

I don’t keep one of them anymore. This is FTSE250 financial services specialist Just a group. I saw a 170% gain after news of a private equity buyout broke on July 30, sending the stock price up 70% in the morning.

Construction company Costain Group this is another success that climbed to 142% on my watch. Having recently secured a £1 billion contract at Sellafield Nuclear Power Station, I believe there is still a lot of work to be done.

Then there it is Rolls-Royce holdingsa 130% augment in my SIPP (and more in my Stocks and Shares ISA) while Lloyds Banking Group generated a total return of approximately 115% after reinvestment of dividends.

That’s the thrill of buying individual UK shares rather than simply following the market. Especially since I also took strange beatings Ocado Groupa decrease of 42%, and Diageoa decrease of 36%. But the winners far outweigh the losers.

All shares of Grupa 3i are shares

Among the “double money” success stories there is one FTSE100 wrestling stands out. Private equity and infrastructure specialist Group 3i (LSE: III).

It is one of the oldest companies in the industry, founded after World War II, and has made money by acquiring, improving and selling businesses for decades. However, few investors seem to talk about it.

I’m glad I’m the exception. I bet on great success with an investment fund and it is now my second largest SIPP holding, after tracker funds in the US. It has increased by a modest 30% in the last year, but by 300% in three years. Personally, I’m up 112%.

In full-year results published in April, 3i reported a total return of £5.05 billion, representing a 25% gain on the opening of shareholder funds. Its star holding, Dutch discount retailer Action, brought in £4.55 billion of that, with revenues up 22%. According to the June update, Action’s comparable sales increased another 6.9% over 25 weeks, with 111 up-to-date stores opening.

Concentrated power

Action now completely dominates 3i’s portfolio, accounting for approximately 70% of its total net asset value. This actually concentrates the risk.

The Europe-focused retailer continues its aggressive expansion with the opening of its 3,000th store and a robust start to operations in Switzerland. It is clear that he has a proven model. But 3i CEO Simon Borrows warns that “uncertain” economic and geopolitical prospects make conditions more challenging. This also makes 3i more cautious with up-to-date transactions. So it looks like Action will remain a star for a while.

Is it worth considering purchasing?

On October 2, broker UBS upgraded 3i from neutral to buy and raised its price target to 4,700 pence, approximately 10% above where it is currently trading.

I doubt that 3i will repeat its last great performance any time soon. Much of Action’s story is value-driven, and valuing stocks is challenging. The trust invests at a 60% premium to net asset value, but this has never deterred investors before.

Overall, life in the private equity market has become tough as higher borrowing costs and profitable exits have become more tough to achieve with investor caution.

It’s still one of my best decisions and I plan to continue making it for decades to come. New investors may consider buying, but only if they understand what they’re getting – a successful private equity engine that depends heavily on one extraordinary retailer.