Neck of lamb after (Spring)?

Probably because leg of lamb is more popular, therefore it's worthwhile importing from New Zealand when we are not producing lamb in the UK.

Neck of lamb is less popular, so not worth importing as it won't sell, so they only have it in the shops when UK-produced lamb is in season.

I think it's classed as "lamb" until it's up to a year old, and perhaps legs are still classed as good cuts of meat whereas the neck might not be (although I think you should have the option to use all of the animal regardless of what one person says is a good cut or not) so maybe that's the reason? Just guessing though

You can still get neck - though it can be a bit harder to find. A good independent butcher should have no problem sourcing it. If that fails - buy it when you can and freeze it.

SORRY can't help but I could not resist this one

Went into the butchers and asked if he has sheep's legs he said no its just the way he walks

Sorry again