This is what I do for a living & reckon you are going about it in the wrong manner by notifying him & paying him as you did.
If I was in his position I would have preferred you spoke to me about it & I would have offered to rectify the faults if I felt they were my fault, even if they were not I have still done what was required if it was not too costly to me.
Putting things in writing seems aggressive to me, you cannot beat talking face to face about a dispute,if that does not work then put it in writing. The rad size is not a big thing, I'm sure he would have fitted it foc if you supplied a new one to replace the one he fitted, after all they're not expensive & it cannot be his fault if he followed the spec on your drawings.
The skip thing, well it may or may not have been his fault, you need to decide with him, who hired it , you or him? did you say where to put it? Is it just a couple of slabs or is it tarmac? would it be costly to put right? do you really think it is his fault? why did you not state your concern at the time or mention it as soon as you noticed?
If you really feel it is his fault, then stick to your guns, if you feel it may be partly your fault & they guy has done a good job, give him a break.
Getting things ironed out at the end of a job all depends on your relationship with him throughout the job, if you were a nob he'll likely tell you to go forth & multiply & shove your grand where the sun don't shine, but if you had been nice I'm sure he would bend over backwards to help you, that's how I look at it, I've never had to experience the first example.
I always suss out my customers during our first meet & if they appear to be nobs, I don't hold back with the price, generally I don't get the job for the obvious reason, but if I do, it would more than cover me for the reason you first stated.
I do also consider that if I don't get a job it could be that they think, I'm a nob, after all it's a two way street.
If the next time you get a quote & you feel it's excessive, you now know why. 😆 Always be nice to your tradesmen, ask, don't demand, unless they take piss.