After months of assessments, we finally got to see the Wizard of Oz CAMHS Psychiatrist, today and he confirmed that DS1 did present as having ADHD in addition to his autism. He agreed with me that any flavour of methylphenidate might not be a good fit for DS1, given his anxiety issues, so he's going to be on a trial of Strattera, starting off with 10mg, each morning.
The "mental health worker" who we'd been seeing said DS1 was definitely one of his more interesting cases. Not half!