Black mould behind kitchen units - who do I call?

This is only likely to be mildew caused by damp conditions. Unless you have a breathing issue it is more unsightly than anything else. The best way to get rid of it is to get some spray bleach, I get it from the pound shops, and spray the area well. It's usually gone in a few minutes. Then get one of those little dehumidifier kits you put the powder in and put it near the effected area. Check it and empty any water out every few days. If you can open a window as high up in the room as you can too, that will help let steam out when you cook/wash/etc. Very easy to deal with and should cost less than a fiver! You don't need a tradesman to spray some spray bleach about, put a dehumidifying tub near the mold and open a window.

It caused by lack of ventilation behind your cupboards if vents had been installed prior to the units being fitted it would have prevented it,black mould will spread and keep comming back until you vent the area. Good luck alex

I agree with the problem being lack of ventilation. This is typical behind kitchen units where the plinths and work tops are silicone sealed and where no vents are in place. Plinth vents and/or a wall vent above DPC level after cleaning with bleach is best solution.