Thanks to Judith in France for suggesting this and giving the recipe.
I've just made the jam and it is wonderful! So here is my version of
Judith's recipe, slightly adapted with everything spelled out for people
like me - this was my first attempt at making jam!

Marrow, Ginger and Lemon Jam Recipe


Ingredients:

* 3 lb chopped marrow
* 6 oz root ginger – chopped fine in a blender
* Juice and zest of 4 small lemons
* 3 lb ordinary white sugar

Method:

Take a big marrow. The more ripe the better as the pulp needs to be
somewhat fibrous. Slice the marrow. Cut off the peel. Cut out the soft
core and seeds and cut into small pieces approx one inch by ¼ inch thick.

Place the pieces of marrow in a large pan with a small amount of water
at the bottom to prevent sticking and steam until tender. Initially
place a lid over the pan to help the marrow to soften and release its
own juice. Stir occasionally to ensure even cooking and that it doesn’t
stick. After the marrow is soft, mash it using a potato masher but don’t
drain off any of the juice.

Add the lemon juice, grated lemon zest, grated root ginger.

Add the sugar, simmer and stir until dissolved. Bring to the boil and
boil gently for about 30 minutes stirring occasionally to prevent
sticking, stir more frequently as the pulp thickens and setting point is
reached.

Test for the setting point by putting a small amount of jam on a saucer
that has been chilled in the freezer.

Pot into hot sterilized jars immediately and cover.

The recipe makes about 4 lbs (2 kg) of jam.

--
David in Normandy.
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David in Normandy wrote:
[...]
After the marrow is soft, mash it using a potato
masher but don’t drain off any of the juice.

Or you may like the effect of dicing the marrow and not mashing: it
makes rather attractively translucent tasty bits. Pursuing this thought,
I wonder if chunkier pieces could be made into a syrupy preserve: if it
worked, this could be a nice little pudding with a dollop of ice cream
or fromage frais.

--
Mike.