'Presumably Leuchars would provide radar vectors to the ILS on request if required and effectively act as the approach controllers for Dundee?'
No Leuchars do not provide that service as Dundee is procedural and send you o/h the DND at 3000' then to dnd tower. They'll provide the LARS from around 40nms out, outside CAS obviously. Although the AIP says 24h, I don't imagine they will provide a basic/traffic service to anything past 2200L unless there was specific reason to... E.g. Emergencies/SAR cover.
I've always found them very helpful towards GA, especially at weekends when it is clearly 9999's blue sky and you can hear them working their ass off for 9-10hours with all us going for our £100 burgers at Fife.