More than a spanner needed in the works
Lured by stories of pound;70,000-a-year salaries for those in shortage trades, thousands of people have enrolled at local colleges for a quick course in plumbing and then set themselves up as qualified trades people.
Mr Marshall says: "It takes three or four years to become a trades person.
These short courses may be good as an introduction but often the training is not much above basic DIY standard.
"We are talking about tens of thousands of people who are starting their training and not completing it. Instead of going on to a modern apprenticeship these people are part-trained and part-qualified, which is more dangerous than being untrained and unqualified."
In a bid to tackle the problem, Summit Skills has established a steering group to look into the possible introduction of a special licence to identify qualified plumbers, but a meeting last week failed to produce an agreement.
Mr Marshall says: "There seems to be little appetite for a voluntary licence scheme because it would be difficult to implement and to police, and the Government shows no interest in a statutory scheme.
"The broad outcome is 'Not yet' rather than 'No'. We don't want semi-skilled or unskilled people working on our systems. If I employed a plumber to fit a bathroom, I would want to make sure he had completed an advanced modern apprenticeship and had at least an NVQ level 3.
"There is a belief that becoming skilled can be achieved quickly, but it is complicated and it takes time. A lot of underpinning technical knowledge is needed as well as the ability to wield a spanner."
Mr Marshall says more than 100,000 existing qualified plumbers need to update their skills in the light of new regulations that will come into force next year, and that colleges will be in the front line to do this.
The rule changes are linked to the fact that from next year boilers must satisfy a new set of energy efficiency and emissions regulations.
"There is a huge issue about upskilling to be tackled," he says. "A massive training programme began last August and colleges are the key training providers."