By site plumbers do you mean gas safe registered plumbers on site or just normal plumbers on 1k a week?Far too many course Cowboys & career change chancers in the gas industry. At one time a RGI could earn twice as much doing C/H installs as a site Plumber. Not any more, the money for heat installs is rubbish & site Plumbers are making £1K/week.
Doing 6 rads & a combi in a day is cr.p money now.
As for boiler repair, maintenance & service work, it's good money but you do need to be highly skilled & very experienced.
I've done all sorts & the best money I've ever made was on bulk LPG work.
Money has not been as good for years doing bog standard house bashing. There's a bit of a Labour shortage in this type of work, guys with the quallies & experience are making a fortune.