Presuming these are Zwartbles girls? I'd have thought they'd be likely to be well enough grown to cope. Is Dinky pure Zwartbles too?
We had unexpected hoggs lamb mid-May one year (rig had worked when temp fell below -15C.). 10 lambed 11 lambs, all fine apart from the twin-bearer. They'd had no feed because we hadn't realised any were in lamb, so the twin-bearer suffered, but all the singles were absolutely fine. So fine, in fact, that we contemplated tupping a batch of hoggs for May lambing every year thereafter! (But can't guarantee singles, and don't like hoggs with twins.). These were commercials.
In your position, with Zwartbles, I'd probably talk to the vet about morning after treatment. If no go, then definitley scan, and I would cake the twins but not the singles after the scan, unless any singles are skinny, of course. And give minerals and so on, of course.
If Dinky is a Zanx Loagwhart, I might think it's fine to let the girls get on with it. The lambs shouldn't be over-taxing and the girls will learn their job. Although Easter Sunday 2018 is 1st April, so you'd be looking to get an 11 month annual cycle from them, notwithstanding the temptation of the date... :/