I agree with other posters that it sounds like something else was at play with your sheep - Toxo? It is well worth finding out, as you may have to vaccinate for it in coming years.
I would like to point out, however that outdoor laming with minimal interference is just that - either lamb inside, intensively or outside, extensively dont half-do one or the other.
If you continue to lamb outside (and I see no reason why not with hill breeds) then learn to leave them alone - I cannot stress that enough.
Lamb in a field with plenty of natural shelter, at a time of year when it is at least 10C in the daytime and you should be fine with the right sheep. Don't be tempted to bring them in because it is raining, for example when they are already under a hedge or curled up with mum, you'll just end stirring the lot up and make a bad job out of a good one. Bringing outdoor born, wet sheep indoors is just asking for disease, imo.
The more that I lamb out, the more fastidious I get about not stirring the ewes up. I creep round the edges like I was shooting rabbits or something, crook in hand, lube etc in my game bag and if I should see any laming and I simply have to watch, I do it through binoculars. I get really narked when I bring people round and they just march through the middle of the field and/or turn up in bright/loud clothing. If I am marking them up/tagging then causing general pandemonium by bowling through the middle of them means I wouldn't ever catch up with some.
If I were you, I would try and pinpoint your problem, either medical or genetic and then make a decision. As far as I am aware, Welsh Mountains damn well ought to lamb outside and any that don't (underlying medical issues aside) should be off down the road asap. When you bought your ewes, did you buy them off an outdoor lamber?
I remain convinced that lambing outdoors is a very successful way to breed sheep but it is fundamental that you start with the right sheep in the first place. For example... I am just coming to the end of my first mob of outdoor lambers (100 ewes). So far I have had 1 orphan and a couple of stillborns. Lambing at almost 180% (easycare x charollais) at a farm about 12 miles away from me.