Cheers Dan.
Can I ask you guys if £20 a m2 is ok? or am I far too low there. What could/should I be charging for large format, mosaics etc.
It's not uncommon to see your standard ceramic price to increase by anything between half again (50% more) or double when fixing natural stone, large format tiles of most materials but especially stone, trav, marble, even some porcelains.
Though I'd also consider this as being a trade job as the lead is coming from the plumber and you don't have all the time quoting (and you don't get all jobs so even quoting on three jobs yourself you may only get one, could even be a days work quoting for the three on that sort of job depending on their locations etc!) and you're not spending the time with the customers selling to them etc.
Though you can't work for nothing, so this does need sorting out, I'd just work on getting the plumber to understand what he pricing and what the job is worth appear to be totally different things here!