Bed bugs are a sod to get rid of.. they often take several treatments to fully resolve the problem - I would suggest that you get some sort of guarantee from the pest controller who takes on the job, as it is can be an absolute marathon of a task and rarely resolved by just one visit. You may also be asked to dispose of the mattress, bed and bed sheets, so you may want to let the client know that that is may happen.
Are you a pest control company as well? If not, I wouldn't advise tackling it with products from your local hardware store.. as you'll probably guess, if its available to the general public, its probably not very good.
You could speak to the British Pest Control Association - they'd be a good bet.
Luke