Confidence - as distinct from arrogance - is not something Costerton lacks. But having ascended to one of UK PR's primary posts, there is no reason not to be bullish. H&K has clinched some major new accounts of late, most notably Intel and Pepsi. Nevertheless five years ago, albeit in different market conditions, H&K had 100 more staff than it currently employs.
Costerton, still officially head of technology EMEA, replaces outgoing CEO Marie Louise Windeler, who will leave at Easter after 20 years at the company. In spite of her promotion, Costerton will maintain a hands-on role for the mammoth Intel contract H&K secured last month.
Right now, she is keen to outline her plans for graduate employment at H&K. These include introducing a 'rotation system' for young PROs at the agency to find which discipline best suits them. 'We've always invested in graduates but I want to increase that commitment. PR is a people business. We literally have nothing else,' she argues.
She believes ardently that the industry needs to improve its own reputation for robust training processes in order to compete effectively for graduates against 'the McKinseys and Goldman Sachs of this world'.
This means challenging the image of an industry manifest in the public consciousness by Max Clifford and TV comedies Absolutely Fabulous and Absolute Power.
'We shouldn't sit around and moan about that image,' says Costerton . 'We should be the ones saying what PR is about. I'm not trying to boil the ocean, but PR should have a more visible role in UK plc.'
She says the low point of her career was her underestimation of the knowledge needed to smoothly move from QBO to technology specialist Abacus (now Kaizo) in 1995.
A couple of weeks' pretending to understand computing jargon was eventually addressed with a notepad and long evenings with a dictionary.
Costerton says her mantra when dealing with clients is to ask: 'What is success for you?' Although fluent in argument nowadays, she admits it is a poser she would not have been able to answer as a graduate.
'I fell into PR by mistake,' she recalls. 'I've never had a set plan.'
Her rise has been steady and impressive. She moved from a graduate job in 1988 to join QBO. After Kaizo (during which time the agency relocated from Camberley to London) she joined H&K in 1999 as technology MD.
Costerton's first PR experience though came inadvertently at the University of London, where she ran the union's rag committee. She claims her biggest coup was persuading The Smiths to play at the student union. They tried to pull out but eventually honoured the gig - getting Morrissey to do something he did not want to was perhaps an early sign of Costerton's negotiating skills.
These days, thanks to the influence of both her 12 and 14-year-old children, the jazz and Mozart on Costerton's iPod are accompanied by the Kaiser Chiefs.
Given the inevitable increase in workload, she consulted her family before accepting responsibility for 250 staff. And she says she has promised to find the time to cook at their Surrey home at least twice a week.
Costerton's nursing of H&K during its difficult period after the dotcom crash, handling redundancies while steering its focus away from B2B to consumer technology, demonstrated firm leadership. And the powers-that-be at H&K have not forgotten this.
'I think we are entering a new era,' she says. 'I've spoken a lot about what I want to do with the agency and have complete backing from H&K.'
Costerton, even before she officially steps into her new role, is most definitely in charge.