OK ...M and S have now sent me a reply ...interestingly it expands on the halal lamb and NZ!
"Generally speaking all of our fresh meat is produced in the UK, but for ready meals we tend to use meat produced in the EU. Marks & Spencer is committed to the highest standards of animal welfare and this applies to all the livestock used for the production of our food products. Our livestock specifications cover the welfare of animals throughout their lives - on the farms, during transportation and at the time of slaughter. All abattoirs used by Marks & Spencer, both in the UK and EU are known to us and are audited to ensure they meet our specifications for animal welfare, traceability, hygiene and food safety.
Our requirements for the humane slaughter of animals insist that all animals are effectively stunned as part of the slaughter process. We do not sell the meat from any animal or bird that has not been pre-stunned. Our supplying abattoirs employ Animal or Poultry Welfare Officers who are specially trained and responsible for overseeing the animal welfare requirements within the abattoir. In addition, the abattoirs employ veterinary surgeons who, in addition to meat hygiene inspection, are responsible for ensuring the humane slaughter of animals.
Regarding your concerns about halal slaughter I would like to reassure you that, with the exception of New Zealand lamb, none of our meat or poultry is slaughtered in accordance with any religious practices. The New Zealand lamb industry is heavily reliant on the export of lamb throughout the world. Many of the countries to which it exports have Muslim communities and as such all New Zealand lamb is approved by a halal authority.
We understand that for some there may be a religious or moral objection to New Zealand lamb, so we’re able to offer UK sourced lamb as an alternative. The country of origin of all M&S meat is clearly labelled on the packaging."
I, for one, will be looking closely at the packaging for source...this one reads that all NZ lamb is slaughtered in accordance with the halal authourity ...so would seem doesn't matter whether it is fresh or frozen.