The FTSE 100 crept higher, but the day's big movers were outside the blue chips, as 'mid-cap' stocks Spire Healthcare and IWG both lost more than a fifth of their value.
The UK blue-chip index rose five points higher to 7,663, gaining only modest support from a fall in the pound, down more than half a cent against the dollar at $1.294.
A weaker pound tends to support the FTSE 100 as its stocks rely on overseas markets for around three-quarters of their earnings.
On the FTSE 250, Spire Healthcare (SPI) slumped 21.8% to 193.4p as the healthcare firm warned of sharply lower profits due to lower referrals from the National Health Service. Revenue from the NHS fell 9.5% in the first half of the year.
'The current difficult market conditions - also seen by other operators - had a greater impact on our business... than we had expected,' said chief executive Justin Ash.
Jefferies analyst James Vane-Tempest said expectations of 2018 earnings were likely to fall by around 15% as a result.
'It is clear that NHS business continues to be weak for Spire and there is evidence that further NHS triaging and rationing should impact into the second half,' he said.
Neil Woodford, once the company's largest fund manager backer, avoided today's share price slump, having sold out of the shares earlier this year.
Woodford had invested in Spire shortly after the launch of his Woodford Equity Income fund when the company floated on the stock market at 210p per share in July 2014.
He held over 16% of the company's shares as recently as March, but steadily sold of his stake, which fell below 5% in June.
Joining Spire at the bottom of the FTSE 250 was IWG (IWG), 20.5% lower at 238.5p as the serviced offices provider abandoned takeover talks with three private equity suitors.
IWG said it did not believe Starwood Capital, TDR Capital or Terra Firma could offer a price it would be able to recommend to its shareholders.
The company also reported a 33% fall in pre-tax profits for the first half of the year.