Have I really stumped everybody ? As Victor Meldrew said - " I DON'T BELIEVE IT "

Not really.
Seed packets tell you what you are growing.

As long as the seeds are not F1 hybrids you can save them.F1 hybrids seeds are not the same as the parent plant which is why you can't save them.

However, if you grow different varieties of heirlooms, which basically are out of favour original old fashioned varieties, or varieties like your Marmande, close together cross pollination may occur meaning you may end up with something very different.....and that is how F1 hybrids eventually are produced.

So yes, you can save Marmande seeds as they are normal cross pollinated tomatoes.

Most tomatoes are self fertile and non hybrid ones cross exactly, so Marmande would be one that reproduces itself year after year.
Even with F1 hybrids, a large number of the F2 generation will be very close to the parent, and some people have taken F1 types, bred the F2, and then 'stabilised' it into a self fertile variety which is as good as the F1. Sungold comes to mind as an example, with Sungella as the outcome (or maybe the other way round?) which was done by an amateur. If you have space to grow lots of the F2 generation and then select the best with the characteristics you want, you can do it too.
I would thoroughly recommend the book: "Breed Your Own Vegetable Varieties" by Carol Deppe, by Chelsea Green Publishing, USA. Far more amateurs should be attempting their own vegetable breeding.

F2 hybrids are very hit and miss.
At the moment I am playing around with an F1 sweet pea that was pink. On saving seeds my F2 plants have been white, maroon, and pink, from the same plant. It will take a good 5 years to stabilise the F2.

Hybridisation is a fancy name for what nature does on it's own.

Ah, there you are you guys ! Teasing me were you ? I knew you'd know the answer, but thought you were perhaps a bit fed up with my puerile questions.
As I've said more than once, I still consider myself a beginner and a very late one at that. But thanks very much, I have a huge Marmande tomato and will get on with the job today.