Hello Declan. Some of these pure breeds have a habit of being very nimble. I had a couple of hardy Welsh mountain sheep and they were exactly the same. If they could not jump the fence, which they invariably could, they'd make a hole in it - I sold them in the end as I was sick of getting phone calls to say they were in someone else's field!!
Anyway, I know how you feel as regards the shape your sheep are in. I have fed and fed and fed and its cost me a fortune in good hay and dried feed (coarse ration and nuts) and mineral licks and my ram lambs are still thin. My ewes have just lambed and a couple of them are looking thin too. Heaven only knows why because they've been fed everyday for months, and for the last four weeks my ewes have been fed twice a day with dried food with unlimited hay. It is disheartening I know, but I put it down to the bad winter and hope that the new spring grass will sort it.