There are going to be quite a few plum posts, as I seem to have gained an embarrassing excess.
This is a simple "technique" jelly, in which sliced plums are held in a plum jelly. The recipe calls for the plums to be simmered in Riesling, but I switched it to half water half wine, mainly because that was the amount of wine that was spare. Halve & stone you plums, add sugar to taste cover with wine/water and simmer for 10-15 minutes. Push the mixture through a fine sieve into a jar & make up to 500ml with water. Do the usual jelly routine of softening & melting the gelatine. Line your mould with halved plums and then pour in enough jelly mixture to just cover them. Fresh fruit floats in jelly, so if you want it at the top of your decanted jelly you have to let the first pour thicken. If you stand you mould(s) in a bowl of ice water the process goes that bit quicker.
After an hour or so the plum layer should be gelled enough to add the rest. Due to the slightly magical nature of gelatine, the top should set faster then you'd expect. Chill for a couple of hours & then serve. I didn't unmould this one, serving it instead in glasses, but it looked good & tasted great (quite a grownup taste due to the sourness that comes with plums)