Supermarket promotions Q4 2021

I am undoubtedly tedious on the subject but in 1960 a bottle of Mumm Cordon Rouge NV was the same price as a bottle of La Tache 1952. Champagne has never been cheaper.
Last time I had Cordon Rouge it was more than decent. It was a couple of years ago though.
I'd be fascinated to taste an NV as it was in 1960, in pictures it's always dark yellow.
I've seen a rumour that the current Sainsbury promotion will now run to 19 December.

Morrisons have 25% off 3+ on their their 'best' own label range, I think until 9 December. The highlight is a 2010 English fizz, producer unknown £25 -> £18.75. I believe Blue Light card holders get a further 10% discount and some bank accounts are also giving rebates on Morrisons spend.
Is it possible to find out what shops have stock of certain wines?
For JS, add the wines you're interested in, reserve a click and collect order and it should show wines not ranged in that store as 'unavailable'. You might have to re-add to your basket if you need to try other stores.

Alternatively, the CS line should be able to tell you whether an wine is ranged in a particular store.

Neither route will tell you actual stock levels, not least because someone may have bought the stock between checking and arriving at a store.
So Sainsbury's told me that any "fine wine" level Champagnes are entirely out of stock at all stores, they haven't been delisted, but are "on-hold" pending resolution of "Brexit" and "Covid-19" challenges (no political responses please, I am just quoting).