As most trades in local areas do, Plumbers usually advertise in local magazines, yellow pages and sometimes door to door flyer for their business marketing. This strategy has paid off in the past and can also be enhanced by having a website to refer people on to once they have seen the ad or flyer.
A few photos showing some portfolio work, such as bathrooms would give the viewer some trust in previous work. These images could also include trade vans, premises and team shots.It would be a good idea to have certification images from accredited bodies like ACS Gas Safe, old CORGI (as people still look for this!), checkatrade and anything else that shows the plumber to be good to hire.
A contact form could send an enquiry to a specific email address that could be dealt with by office staff for a larger organisation or go to an inbox on a smart phone, so that the plumber can respond whilst away from the office.
Keeping track of old customers and reminding them of your presence with offers of health-checks, follow ups or discounts on parts are all suggestions that would work really well with a plumbers promotional strategy.
A plumber is often limited to a particular area due to traveling time & fuel expense. This preferred radius would be a local search and could be matched to what people are searching for online.