LONDON -- The FTSE 100 had an erratic month, being blown by the prevailing political winds. But it rallied toward the end of December to hit an intraday high of 5,997 points, finally setting a new 52-week high. The index of top U.K. stocks has since gone on to break the long-awaited (but not actually important) 6,000 level, standing at 6,034 at the time of writing.

December also brought some nice gains from individual shares, and here we take a look at five that you would have done well to have bought. But they're not just the biggest risers; they're companies that look like they could be in for a decent long-term spell.

RBS (NWG 1.36%)
Investors are finally regaining their confidence in the banking sector, with even Royal Bank of Scotland rebounding strongly. During December, shares in the bailed-out bank gained 29 pence for a 10% rise to 325 pence, and that takes the price up 60% since the summer. The other taxpayer-owned bank, Lloyds Banking Group, has done better overall, gaining more than 80% over a similar period, but December belonged to RBS.

With RBS's return to profit being so recent, the current price-to-earnings ratio of 21 might look a bit high, but that will surely decrease as profit growth resumes over the next couple of years. There is no return to meaningful dividends expected yet, but that will be an important milestone.

Barratt (BDEV -0.09%)
Barratt Developments made the list for September, having gained 11% that month, and since then the price has carried on upward. It gained 6.6% in December to 208 pence, taking the rise further since that last report to 22%. The shares are now up more than 120% over the past 12 months.

Is it too late to get a share of the recovery profits? Well, with all of our major homebuilders having bought up development land when it was cheap during the recession, I wouldn't be at all surprised to see a decent five years ahead for the industry.

Thomas Cook (TCG)
The impressive recovery at Thomas Cook continues unabated, with shares in the travel agent putting on 22 pence in December for a massive 85% gain to reach 48 pence -- and the price is up another couple of pennies this week to 50 pence. That really is the kind of gain you don't get very often, so congratulations to those who managed to time it right.

What about the long term? Well, November's full-year results looked pretty positive, marking a significant reduction in net debt and a modest underlying profit. But there's still some risk for sure, and a couple of decent booking seasons would not go amiss.

Next (NXT -1.04%)
Finding the companies that will do well in the Christmas trading period is an annual challenge, and you could have done worse than to put some money on high-street fashion retailer Next this time around.

During December, Next shares put on a modest 48 pence to finish the month at 3,709 pence, but that ended a strong year that brought a 36% rise overall. And this week we have seen a further 4% rise to 3,847 pence on news that trading at Next during November and December has been strong, leading the firm to lift its profit guidance again.

ASOS (ASC -1.21%)
Online fashion success story ASOS has had a few ups and downs, but in December we saw a continuation of a strong recent rise. The price climbed 8.6% during the month to end the year on 2,691 pence, and in the past few days it has put on a further 31 pence to reach 2,722 pence. The price has now more than doubled over the past 12 months and has beaten its early 2011 peak.

Forecasts for the year to August 2013 put ASOS on a forward P/E of about 55, which assumes a fair bit of growth still to come. That might well be there, but the growth investor crowd is a fickle one, and all it might take for another price reversal is for ASOS to narrowly fall short of some lofty expectations. Still, a majority of City analysts rate the shares a "strong buy," for what that's worth.

Some of the shares discussed here are quite risky, and many long-term investors prefer to go for dependable, dividend-paying shares. Neil Woodford, who regularly beats the FTSE, is an acknowledged expert at the dividend game, and Motley Fool analysts have put together the free report "8 Shares Held By Britain's Super Investor" to delve into some of his major holdings. Click here to get your free copy while it's still available, and it will be delivered directly to your inbox.