Just a quick update for anyone in the same position, I had a look around the cheapest equivalent I could find was Quinn's CompanyCare Starter. From what I could see, it's exactly the same as what the family has but is about €50 cheaper.
VHI's Company Plan Starter was just over €100 cheaper, but the out-patient cover was more limited (limit of 4 each visits to a GP, alternative therapists, etc). They use chiropractors and GPs a bit so it wasn't really going to work out in the long run. Also, there's no cover for private hospitals on this plan, in case anyone else is looking at it.
Oh, and First Plan Plus wasn't cheaper, I'd forgotten the second child!