A few points on what you've said from someone who's been down this route;
1) You state the field was cut for hay for 50 odd years, that sets alarm bells ringing as that basically means that most of the trace and key mineral elements will have been removed from site - I started making my move to grass-only minimal fodder sheep keeping after realising this, that the feed value of alot of meadow hay is only marginally better than barley straw (which is dirt cheap) - and that the trace mineral deficiency issue was being made worse by me feeding the animals a deficient fodder crop.
2) Your keeping w.woodlands on lowland grass - they're not in the right place, I've had some myself (as drafts) and they lean towards lambing issues, over fat, the lambs are poor grading and fatty if bred pure on soft grass - they are adapted to the bleakest wetest and coldest parts of the high pennines, that said, they will be less likely to suffer mineral issues as they are adapted to scarcity - they ARE vulnerable to copper, as a result of this adaption. (I know of one man who brought 40 draft hill sheep, a mix of swale and w.woodlands and the death rate was 18 on fields with high copper due to pig slurry being spread - same fields harmless to Suffolk crosses).
3) Personally I would suggest getting the soil and grass tested - start point - and try and find a local source of manure and lime, get the ph as close to 6 as you can and get the PK at least 1.5 and abouve - the difference this makes to how well sheep "do" is amazing, from better fleece, immune systems, to growth rates.
Then look at sources of trace elements - simply feeding fodder beet out can help their - and the minerals pass through the sheep and enter the ground.
The key is also cost - trying to do it strictly local, and in an idealistic "traditional" manner will bankrupt you! Its better to look sustainability as a long term thing - get your soil right at the beginning, get your sheep breed right for your soil, and long term you will need few if any inputs.
Me, personally - Im farming 72ac lowland grass in the ph range 4.8-6.9 (rented ground poorer than owned) and keep mostly Texel & Easycare x Hebridean ewes - as this cross combines thrift and low food quality and volume requirements of the hebridean, with a commercially viable meat carcase and fast growth rates. Similar could be done with your W.woolands - taking their feed efficiency, hardiness and combining that with a faster growing commercial beast, combined with growing the best grass you can.