You won't make you a bean like that. You are giving leads away but can't work out why people don't want to buy them instead.
Give half a dozen out as a free sample to serious buyers maybe, but you ain't a charity for low income plumbers.
I would categorise your enquiries into postcode areas first, then quote type:
boiler leads
full house ch leads
heat pump leads
bathroom leads
etc
Then confirm interest, call the punters to check details are correct and see if they are seriously interested in getting quotes for work completed within weeks/months timescale. (do this thoroughly, no use selling weak leads- call back in 6 months or bin them)
Once you have sifted the leads and asked all the right questions, they should all have a good sales conversion rate.
You need to get an idea of what the jobs are worth, say if a condensing boiler job is nearly 2k, A decent lead is worth X% of the order value. The X is negotiable depending upon order quantity, supply/demand of leads, how busy local tradesman are and
your business and negotiation skills.
There are a few leads companies on here, regularly plugging themselves ad nauseam, but they are all seemingly far too important to respond to pm's unless one is ordering several hundred leads in several postcodes weekly...