Hi Gash - sounds like your son found some good stuff there.
At least two of these are Clarecraft figures. Clarecraft no longer exist, but they made an extensive range of Discworld figures.
I'm no expert on these, but the Detritus figure is (I think) half of a set of bookends - there should be a figure of Ruby to go with it. I have no idea how that would affect the value though.
The DW 07 sounds like a figure of Nanny Ogg. This one
sold on eBay for £23.
The other figure, while being Clarecraft, isn't (as I don't think) a Discworld figure. I think it's called Wanton Witch and is part of their 'Faerie Realm' series. I think it might be another case of half a bookend.
As to value - well that very much depends on what people are willing to spend.
I think he did really well to find those at such cheap prices.