If you are giving them feed then they are getting some minerals in those. (I should have thought of that with it being dairy sheep.

) So they may be okay with just their cake and a lick, but you'd have to get the vet to do bloods to be sure.
Fleece is a good indicator of cobalt and copper, though - if the fleece is tender, miscoloured, has a harsh feel you wouldn't expect, any of those things would indicate mineral deficiencies. How were their fleeces this year? (And did you save one for me?

)
A lot of us use Jonathan Guy Animal Health for our drenches; he is very good and will make up smallish quantities, hence his popularity with smallholders. I would suggest you chat (or e-chat) with him about your situation and see what he recommends.
I've not used boluses in sheep myself, but several on here do or have. Have a search on "bolus" in "Sheep" and come back and ask if you still have questions. (And indeed, to tell us what you did and how it worked out for you - it's always good to know the outcomes!)