Foinaven - good to see I'm not alone in the path I plodded down! I too am stuck, but the bit that worries me is the move from 12a to 13d. I eventually went down that route, but surely it leaves 6dn 'orphaned' as it leaves only the junction with 4a as a potential change, and there need to be two changes per answer?
I can see an alternative way forward from 12a to 6d then 4a, 20d, 7a and 15d, BUT it relies on a re- word which is not given in the BRB but is listed on QAT (union).