David Martinson still has not found what he is looking for - and he has been searching for almost three years.

Not that this is a bad result because David is a laboratory co-ordinator at Southern Water's Falmer lab, tasked with testing water to make sure it does not contain cryptosporidium, a tiny micro-organism that can cause illness.

Regulations require water companies to sample continuously and analyse for it daily in the treated water at potentially vulnerable sites.

David, 50, said: "The majority of our water comes from bore holes, which means it is typically very clean but we're required to regularly check 11 sites.

"We've been testing since April 2000 and haven't found any evidence of the protozoan parasite."

He is one of about 30 staff at Falmer dedicated to water testing, the majority of whom are qualified scientists.

David spent seven years studying and researching analytical chemistry and microbiology to doctorate level before joining Southern Water 24 years ago.

He said: "Most of our work is routine and potentially could be boring but that doesn't mean it is easy. The science is complex and the water-testing process is quite technical."

The lab, which conducted about 380,000 tests last year, is open 365 days a year.

David said: "There are at least five staff here every day, making sure drinking water is microbiologically safe. That's different from what I do but it's all about keeping the water bug-free.

"We also have standby analysts who can be called at any time. For example, if a tanker crashes and spills diesel it might get into the water table and that's what we need to check - in the middle of the night if necessary."

David said: "We don't employ 'button-pushers'. If people don't understand the science behind what we're doing they're no use if things go wrong. The good thing is nothing's gone wrong so far."