Lead Generation requires you to put yourself in the mindset of the consumer - do some keyword research to work out what is the search intent based on a need. In your example of "plumbing" trigger keywords might be "boiler repair", "radiator installation", etc.
Gather a list of relevant keywords and run a PPC campaign to get an idea of which keywords actually convert for your client – you might want to use modified broad match to cast your net as wide as possible.
Also you have mentioned onsite issues – make sure you have a clear call to action on each and every page. Personally I am not a big fan of squeeze pages / placing forms on every page. I feel it works better to have a call to action that is fairly noncommittal for users like “order a brochure” – the sales process can start once your client has the contact details. In your plumbing example, most people offer “free quotes” etc. (different in emergency cases).
Whatever your client’s niche – talk through the sales process and make sure you understand what would be the very first step in that process and focus on getting as many people as possible to make that step.
Back to SEO – while the PPC test campaign is running starting building links (try to avoid being too keyword rich in the early stages – especially for new sites – so get a lot of brand links). Also look for link targets – build a list of really high quality sites you want to get links from and begin to formulate a strategy to get those links.
Analyze the PPC data, optimize the site for the most profitable keywords and then start to get the odd link from the high quality sites with your important keywords.