If you knocked on a house door, and the owner said "I don't want to pay more than £5" would you give it the time of day?
A customer like this is going to be trouble. Same with the property maintenance people - they do the profitable bits, but don't want to know about the window cleaning, but at the same time they've already given an "all inclusive" quote, so your price has got to fit in with the pittance that's left after they've had their share.
If you want to work for the minimum rate (if you're lucky, and graft like a lunatic navvy) then wait 6 months for your money, provided they don't withold on some quality pretext, then go for it.
As to what "diffusers" are, have you ever seen those lozenge shaped thing they set in the pavement to give light to cellars beneath? Well, that's one type.