Derek, I'm just going off what I have read elsewhere on the net, that it takes several hours.
Several hours can easily equal 4, which is also 1/2 a day.
I have seen others say that they do it in that time, you may not, its not a problem.
Maybe others are not a thorough as yourself, it was only an example & not a definitive "it only takes xxx time to do".
It's easy enough to find out about it on the web.
And £200+ IS "more than £200" (note the "+") which is what your charges are also (more than £200)
I gave that figure to show that a powerflush is an expensive job.
I would not dream powerflushing as a DIY job, mainly because the equipment is not available for a DIY'r.
However flushing the system out with mains water is & As far as I know better than not doing it at all (so I'm told) and is the DIY option. Not that I would expect the op to do that.
I posted for information, not as direct competition to what you do!