Well all I can say is a massive THANK YOU to all your suggestions!
I put out grit along with some small, freshly cut saplings. Opened the pop holes and gave them 'medicated' feed, not sure what the medication was as I got the feed in pre treated abgs from Wynnstey.
The birds are now fine, no pecking and no other deaths.
This year we'll be making another pen to spread the birds out even more as the most likely cause was pecking.
Thanks again,
Sion