From Section 55 of the Child Support Act 1991:
55 Meaning of “child”.
(b)he is under the age of 19 and receiving full-time education (which is not advanced education)— (Not higher than A level standard).
(i)by attendance at a recognised educational establishment; or .
(ii)elsewhere, if the education is recognised by the Secretary of State;
“recognised educational establishment” means an establishment recognised by the Secretary of State for the purposes of this section as being, or as comparable to, a university, college or school.
I would look at where the child is receiving his education, is it at a recognised educational establishment and whether he attends at least 12 hours of tuition a week, the threshold of being classed as 'full-time'. According to this definition, an apprenticeship does not qualify as it is actually on-site training.
It is unclear what you mean by "placement" whether this is education or on the job training. If the PWC is inelligble for Child Benefit, you probably have a case to stop paying.
Don't listen to what Carillion tell you - do your own research. CSA have been known to give out duff info as well.
It does't matter what the ex tells the CSA, you have to defend your own corner and research matters yourself.
Conduct business in writing with the CSA, that way you have a record of the advice they are giving you and the Rules they are applying.
Tip- the CSA will often quote a Clause from Child Support legislation to back up their decisions. Check this out as sometimes there are adjacent Clauses that will give you a defence.