After a return to Watford on loan in the 2009–10 season, Heiðar played regularly for QPR in 2010–11, as they secured promotion to the Premier League as champions.

[citation needed] In three seasons, Heiðar scored 31 goals in 54 league appearances for Þróttur, his performances generating interest from several European clubs.

[citation needed] Heiðar was signed by Watford manager Graham Taylor midway through the 1999–2000 season for a club record £1.5m.

[citation needed] Back in the First Division, Heiðar was part of a Watford team that was undefeated for the first 15 games of the season, topping the league.

However, when the team's form slumped in late 2000, Heiðar suffered a crisis of confidence – one of several during his first few years at Watford – and was well below his best for a large amount of that campaign.

Inevitably, his goal tally brought about interest from Premier League clubs and with Watford needing money, the offer from Fulham was accepted.

[citation needed] On 20 July 2007, it was announced that Heiðar was leaving Fulham to join Bolton Wanderers for an undisclosed fee, as part of manager Sammy Lee's squad restructuring.

[7] He made his Bolton debut as a substitute for the injured Kevin Davies on 11 August 2007 with Wanderers already trailing to Newcastle United.

[citation needed] On 20 November, it was announced that Heiðar had joined Queens Park Rangers on an emergency loan until the end of January 2009.

After regaining his place in the starting XI, scoring 13 goals as Rangers won the Championship in the 2010–11 season including a late winner against London rivals Crystal Palace at Loftus Road.

These goals proved crucial as Rangers avoided relegation on the final day of the season despite a 3–2 defeat away to Champions Manchester City.

[12] On 15 September 2009, Watford successfully acquired the services of Heiðar, on a three-month loan from Queens Park Rangers,[13] meaning the player returned to the club who he played for between 2000 and 2005.

[18] Heiðar scored his first goal for Cardiff in a pre-season friendly against Bournemouth and his second came from the spot against Northampton Town in the League Cup on 14 August.

Despite helping Cardiff secure promotion to the Premier League, Heiðar left the club when his contract expired at the end of the season.

Heiðar made his debut for Iceland in an April 1999 friendly match against Malta, coming on as a substitute for Ríkharður Daðason, and as of October 2010 has earned 52 caps, scoring 12 goals.

After playing in a friendly against Spain in August 2006, a long spell outside the national team followed during which Heiðar announced his retirement from international football, in June 2007.

[citation needed] On 28 August 2008, Heiðar announced his return to international football and was immediately selected in the Icelandic team to face Norway and Scotland a few days later.