Primma, a nine-year-old, was bought in Belgium three years ago by Stanley and Jean Powell, who are Armstong's parents-in-law. The mare was covered in mud fever at the time and, according to Armstrong, she looked a wreck. She was nevertheless bought because of her obvious scope as a jumper.
With many of the leading riders now jumping in the Netherlands, Armstrong did not have a strong field to beat, but Primma did all that was asked of her, defeating Ireland's Erik Holstein on Ballaseyr Kalosha by 2.45sec.
Armstrong rode another mare, Corella, when winning a team silver medal in the 1993 European Championships. To the rider's great distress, Corella died from a brain haemorrhage just four months later.
Primma may, however, be an even better mare. "She has fantastic scope and heart; she'd take on anything," Armstrong said. He will not be riding her in the Derby here on Sunday, partly because she is so brave that she might leap straight off the top of the 10ft 6in Derby Bank.
Armstrong and Primma gained points in all four of the new team trials held this year, twice finishing fourth in two of them - at Windsor and Peterborough. They were a couple of points adrift when missing automatic selection for the European Championships which went to the top three - Michael Whitaker, Robert Smith and Di Lampard. John Whitaker and Geoff Billington were added after the selectors met on Monday.
Capt John Ledingham, part of the Irish team for the championships, rode Millstreet Ruby to win the Hickstead Salver by a colossal margin of 13.85sec.
BRITISH JUMPING DERBY MEETING (Hickstead, Sussex): Derby Tankard: 1 Primma (M Armstrong, GB) clear, 44.77sec; 2 Ballaseyr Kalosha (E Holstein, Irl) clear, 47.22; 3 Lionel (R Hoekstra, GB) clear, 51.00. Hickstead Salver: 1 Millstreet Ruby (Capt J Ledingham, Irl) clear, 76.55; 2 Comex (W Funnell, GB) clear, 90.40; 3 Burnbrooke Again (G Fletcher, GB) clear, 92.16.Reuse content