How do they work out the validation cut-off when they send you the visa? Is it 6 or 12 months after the medical and police checks?

Its usually 12 months from the meds or police checks, whichever is the earlier of the two. Howevere, if a Health Undertaking is required, then it may be cut to 6 months.

Normally 12mths after police cks/meds. but watch out you could get caught out on the short time. we received are visas and had to have them validated within 4 weeks.