If there is such a thing as an ordinary prop forward, Martyn Madden is not it. It's fairly obvious that he couldn't play anywhere else on the rugby pitch, since he weighs in at 18-and-a-half stone and is as wide as the berth it is wise to give certain parts of Grangetown, his home patch in Cardiff, which is more Ninian than Arms Park.

When he was 'resting' in his early twenties, between leaving St Illtyd's School in the city and resuming his rugby with Rumney RFC on the eastern outskirts, he went up to 23 stone, which must have been a sight to behold. But I bet he maintained that jauntiness of step that tells of quick-twitch muscle beneath the roly-poly.

And that's the thing about Madden. The boy can move. In two games against Leicester in the Heineken Cup he has been the star turn, picking up loose ball, or taking short passes, and exploding forward, neckless head bowed, tree-trunk legs pumping. Everyone was expecting Scott Quinnell to make the hard yards. Who was this loose-head that nobody had ever heard of, going 0-60 in a blur and sending tacklers head over heels?

In fact, he's been around a bit longer than you might think. He says he's 28 but he might be 29. After picking up his career with Rumney, where he was coached by Mike 'Spike' Watkins, the former Wales hooker, he went to Pontypool where David Bishop was player-coach. From there he went to Penzance with Mark Ring, before Gareth Jenkins, whose Llanelli were suffering from a front-row injury crisis, took him on loan at the back end of the 1998 season. All these coaches - and they're all idiosyncratic, if not downright eccentric in their own way - have recognised that Madden is special.

I first met him in the Pontypool club house, half a dozen years ago. He was describing a try he had scored that day: 'I'm running along and I gets the sniff of the line. I shouts, "Bish, Bish, give me the ball." And he passes, which was such a surprise, and not just to me but to everyone one in the team - you knows the Bish - that I nearly drops it, like.'

He shrieked with laughter, words and mirth pouring out in a stream of strangled vowels that only a native of Cardiff can perfect. The nasal congestion of the city's accent is a comedy routine in itself and stands in complete contrast to the bird song of Llanelli.

In fact, the only time the huge grin was wiped off Madden's face was last season in the Heineken Cup. Llanelli had to beat Gloucester away in their final pool game to qualify for the knock-out stages. And they might well have done so if Madden had held a pass only yards from the line. 'That was my worst moment. That was tailor-made for me, a real bread-and-butter ball. I believe I owe the boys for that.'

The boys might already have forgiven him that rare slip. As soon as he arrived at Llanelli it was clear he could do things in loose play that no other front-row forward could even attempt. He warmed up as a loan-player with the Scarlets against Newport and then Cardiff before appearing in the Cup Final against Ebbw Vale, at Ashton Gate in Bristol. In the closing moments, he scored the only try of the match with a dash for the corner. He touched down with defenders piled high on his back.

But can he scrummage? This is the million-dollar question. In a way, his conversion from tight-head to loose last season betrays his own feelings: 'The tight-head is all about scrummaging. He's the cornerstone. The scrummage is only a tool for me. I'm happy on the loose-head. I can get my hands on the ball more.'

So far in Europe he has been solid at the set piece. But when chosen to play for Wales A against Argentina last autumn he found himself up against the best outfit in the world. And he suffered. There again, who wouldn't against the Pumas in a hastily assembled A team?

As far as greater honours are concerned - playing for full Wales - he has to persuade Steve Hansen that he is fit enough. The New Zealander is a disciple of fitness and will discard if targets are not met. He has talked with Madden but appears to be some distance from selecting him.

This is where it gets tricky with this most unusual of players. 'Martyn has... something,' Gareth Jenkins, his Llanelli coach, says. Which is hardly very revealing except that when you give the 'something' the full extension of the Llanelli lilt you know that he means 'something special'. Something very special.

Quite how to harness that something taxes the Llanelli coaching staff. Madden can shift - and all the way through the game until the 80th minute when the mood is with him - but will Leicester attack him for the first time in three meetings this season at the scrum and wear him out there?

Or might Llanelli give him a more discreet role today? Use him more as a decoy than the principal batterer? Just as they did with Scott Quinnell in the quarter-final demolition of Bath.

If Martyn Madden is taxing the grey cells of Llanelli, think what he is doing to Leicester. The Tigers will say, of course, that they're far more interested in their own game than in the threat posed by a single opposition player. But they've seen him coming their way twice, and twice they've struggled to contain him. As both teams know, Madden moves in mysterious ways. The one that cracks the code of this anything-but-ordinary prop forward is surely on its way to the final.

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