The FTSE 250 was down 0.72% at 20,190.94 at 1400 BST.
Construction firm Balfour Beatty gained as it upgraded full-year guidance after interim profits more than doubled and new orders rose 10%.
Hochschild Mining fell sharply as it posted reported first-half revenue of $347.8m, down from $394.8m year-on-year, as adjusted EBITDA fell to $130.5m from $198.5m.
The London-listed firm said its pre-exceptional profit before tax totalled $15.3m for the six months ended 30 June, down from $97.8m, while basic earnings per share slid to one cent from eight cents.
It swung to a basic post-exceptional loss per share of one cent, from earnings of seven cents a year ago.
Essentra shares fell as the company reported a “strong” first-half performance from its continuing operations, with revenue up 17% on a constant currency basis to £340.8m, and adjusted operating profit up 44% to £35.3m.
The FTSE 250 company said its components operation delivered a good performance, with revenue growth of 18.6% on a constant currency basis, and growth driven through price and volume.
Adjusted operating profit in the division increased to £35.9m, and adjusted operating margins expanded to 20.4%.
ASOS shares slumped after the online fashion retailer said that chief operating officer and chief financial officer Mat Dunn will step down from his roles as the company restructures its executive team.
Dunn will continue in his role and as a member of the board until at least the end of October and will remain employed until the end of the year to provide transitional support, ASOS said.
FTSE 250 – Risers
Balfour Beatty (BBY) 317.60p 9.82%
Darktrace (DARK) 538.80p 4.58%
Carnival (CCL) 810.00p 3.87%
Ferrexpo (FXPO) 147.40p 2.50%
Auction Technology Group (ATG) 949.00p 2.48%
CMC Markets (CMCX) 257.00p 1.98%
JTC (JTC) 787.00p 1.94%
Bank of Georgia Group (BGEO) 1,960.00p 1.87%
Energean (ENOG) 1,264.00p 1.77%
Watches of Switzerland Group (WOSG) 910.00p 1.34%
FTSE 250 – Fallers
ASOS (ASC) 880.50p -8.23%
Hochschild Mining (HOC) 75.10p -7.63%
Aston Martin Lagonda Global Holdings (AML) 506.60p -5.17%
Polymetal International (POLY) 209.10p -4.95%
TUI AG Reg Shs (DI) (TUI) 147.75p -4.52%
Essentra (ESNT) 243.50p -4.32%
888 Holdings (DI) (888) 151.30p -4.24%
Wetherspoon (J.D.) (JDW) 559.50p -3.70%
Mitchells & Butlers (MAB) 182.20p -3.55%
Jupiter Fund Management (JUP) 115.20p -3.52%