Maybe this will be some help to you. Wish he would have kept it in the family, but maybe none were interested. This is from wikipedia. There is more.

He retired in the late 1980s and sold the business to Notcutts Garden Centre. In retirement he lectured and wrote, publishing such books as "The Reader's Digest Gardener's Guide to Growing Roses" and "Growing and Displaying Roses".

In 1983 he was awarded the Victoria Medal of Honour of the Royal Horticultural Society

He died aged 91 in 2017 and was buried at St Andrew's Church in Sandford-on-Thames. He had married twice;firstly Sheila Weatherley and secondly Sheila Port and had two daughters.

Maybe you can get his books he wrote and they will have some information in there that is useful.

@Alana8aSC

Thanks so much for the info on the books!

I will order them, along with one he co-wrote specifically about roses for small gardens.

I see that John's son Robert Mattock is apparently still alive and still very much involved with roses:

https://uk.linkedin.com/in/robert-mattock-msc-phd-9498a427

I hope that either through the books or the email to Robert I will end up figuring out their potted rose techniques.

If they closed the business and aren't selling the potting mix, I'm hoping perhaps they might consider sharing the basics of how to make it.

seasiderooftop I am popping in a link here. The author hybridized and showed roses. The only issue is that he lives in the wet in winter cool PNW US, but we don't get any rain for 3 months of summer. It might help. https://tlcfocus.com/paulbarden/main_august2002.html

Thank you @ann beck 8a ruralish WA !

That was a great read, very valuable resource! I will be sure to apply at least some of this to my potting soil mixes!

One of the things I also wish I knew more about is Mattock's root pruning technique, as it is specifically geared towards Laxa rootstock which is the main one used in Europe.

Laxa has a carrot-like taproot and thinner feeder roots around it. Despite all it's advantages as a rootstock, Mattock says Laxa is unfortunately ill-suited for containers, since the taproot will never be able to reach the kind of depth it would need to be truly useful.

From their website, it seems that he was saying to prune back the Laxa taproot quite drastically, to encourage mainly feeder root development, which he says is better suited to pot culture.

I would never attempt this without a detailed how-to. I hope there will be more info in the books.

seasiderooftop I just saw a new youtube video on root pruning container rose...he is from the hot part of California and he tells a little about his soil mix and you can see a little about how the roots of his rose looks. "Rose Repotting . Step by step repotting of a rose bush - Peninsula Rose Society" (Copy and google...sometimes my links to youtube go missing.) Also conifers in our area can be transplanted if the tap root is cut every 2 years...wonder if that works with Laxa and if that would keep it happier in a pot?

So sorry, @ann beck 8a ruralish WA the notification for your comment only appeared now! Thanks, Houzz...

I watched the video, thank you for sharing! Oh wow, that guy was being rougher than I would be with that rose! He just threw the pot to the ground on it's side to loosen the soil out of the pot! Haha, I can tell this guy has done this a million times and knows exactly how much his rose can take. And I never would have thought to use a bread knife but that's a great idea!

I watched his other pruning video, that was good too. Thank you for sharing!

Interesting about the conifers, that is really good to know! I will look into how they do it. Hopefully it helps plants stay healthy in pots for longer. My roses, like my other plants, will probably never go in the ground, so I want to do everything I can to keep them happy in pots for as long as possible.

The conifer video was obsure from a widely respected wholesale conifer grower in the US...they happen to grow near my hometown. It took them a long while to figure out how to deal with tap roots, but once they got it, they became THE seller of specimen conifers to high end garden centers.