bulldog,
im not an expert on child maintenance but my 'rough' understanding is ...
The mother would not be able to claim against you for the 20 year old due to his age. He may be able to make a claim against you but it would depend on the nbr of hours he did on his course each week (ie is it regarded as full or part time).
The 17 year old training as a hair dresser ...not sure on that one.
If i were you i would visit the child maintenace section on Wiki and give the CMEC people a call - its free and they know their stuff.
http://www.wikivorce.com/divorce/Child-Maintenance-Options.htmlI would stay anonymous on the call just to get the info you need.