Hello and welcome to the forum.
Sadly there isn't a huge amount of publicly available information about how to fault find and repair these transmissions.
A franchised dealer would likely fix this by replacing major subassemblies; the cost would be many times the value of a 2009 Panda. There are a very small number of specialist independent garages with the skills needed to work on these subassemblies, but again I'd expect the cost will be disproportionate to the value of the car.
If there were a cheap and simple fix, I'd expect the previous owner would have repaired it before selling. Your chances of fixing this by adding a bit of fluid, or making some small adjustment, are slim indeed.
My own advice has always been that the most cost-effective way to deal with an older dualogic with a problem is to sell it as it is. However little you get for it, you'll likely be better off financially than if you try to repair it.
Right now I'm thinking that its previous owner may have done just that.
After 15 years use, it's likely that every seal in the transmission has worn to some extent, and that one of these has worn sufficiently to trigger the fault you are experiencing. Finding out which one will be like looking for the proverbial needle in a haystack, and the remaining ones will likely be little better, so even if you could find it, just replacing the faulty one isn't likely to produce a lasting repair.
If you are
very determined, have excellent mechanical skills, and top grade tools and facilities, it may be possible to strip the transmission down and rebuild it, replacing all the seals and gaskets.
Third party seal kits are available on ebay for a shade under £200; whether they're any good, or how you'd find the information needed to use them, I don't know. But it'd make a very interesting project for anyone willing to take it on and document their progress on the forum.
