Cardiff Blues 12 Ospreys 16:A late try by all-action number eight Filo Tiatia pushed Ospreys above Munster into top spot in the Magners League after an enthralling Welsh derby at the Arms Park.

The visitors, although struggling at the line-out, carried the greater firepower and did just about enough to complete a Magners League double over the Blues despite trailing for much of the game.

Inevitably, there was little festive spirit about the opening exchanges with both sides hurling themselves into the physical encounters and a string of penalties resulting.

Blues full-back Ben Blair landed the first after just two minutes, Ospreys responding through outhalf Dan Biggar, and both players missed further kicks before Blair restored his side's lead.

A rampaging run by centre Sonny Parker threatened the first try for the Ospreys, but the move broke down when flanker Marty Holah opted for a grubber kick in the home 22 rather than keeping the ball in hand.

The visitors wasted a further opportunity when 19-year-old Biggar slung out a wild pass. Gavin Henson, playing at fullback, did well to retrieve it, but was isolated and conceded a penalty.

Biggar played a key role when the Ospreys finally crossed the line just after the half hour.

His agility and sleight of hand kept the ball alive before Ireland wing Tommy Bowe fed Lee Byrne and the Wales number 15, in a rare outing on the wing, stepped inside flanker Maama Molitika to touch down.

The Blues' own prodigy, 20-year-old Leigh Halfpenny raced through the visitors' defence on a mazy 50-yard gallop but lost his footing just short of the try line.

A drop goal by Nicky Robinson ensured his good work did not go totally unrewarded and gave the home side a narrow half-time lead.

Both sides missed early penalties in the second period before the Ospreys threw down the gauntlet by bringing on Wales outhalf James Hook to replace Biggar.

Cardiff scored next, however, after a remarkable 60-yard break by Tom James had been ended illegally by Byrne, who received a yellow card while Robinson slotted over the penalty.

A long-range kick by Hook kept Ospreys in touch and it needed two despairing tackles by Halfpenny to save the day as the visitors threw themselves into the search for a winning score.

Their sustained pressure was rewarded with barely five minutes left, a superb long pass from Hook finding Tiatia on the left to dive over.

The Ospreys now lead Munster by one point but with an extra game played.