My White Marseilles has a branch pretty much at the base of the main trunk that emerges at 90 degrees to the trunk along the soil line before bending upwards. I'm a bit worried about this branch being at risk of rot due to the close proximity to the damp soil. Should I be concerned about this and if so, what should I do? Won't pruning the branch cause a wound at high risk of infection?

The fig is planted directly in the ground with a bark chip mulch. I've tried to leave a bit of a gap in the mulch to help alleviate the problem, but it's still practically touching the soil. The tree is about 2 years old.

You could air layer it off and have 2 trees. If not, wait until it is dormant and cut it.

Air layer!  Like Tami said.

Even easier than a air layer. Do a simple layer bend down to ground, dig a small indention in soil, scrap the bark cover with soil and cover with a rock to kept it down. It won't rot it will root,  the extra O is good.

Thanks for the suggestions, won't the wound from the cut be an inlet for disease though? Don't want to end up losing it!

Unfortunately don't have enough space for another tree if the branch does take - gardens in London aren't that spacious and I've already got 4! I think I'm even going to have to be brutal with one of those if it doesn't pass muster to make a bit of room!

If the limb is otherwise healthy and there are no obvious pathogens in your soil, a wound should be okay.  If it is already traveling at the ground level, mound some soil over it.  If the bottom of the limb is already in contact with the ground, it has probably started rooting.  Where I am, it takes about six weeks to get a healthy set of roots on my layers while the trees are in growth mode.  Cooler weather/slowing growth will affect the time needed.

Also, given space limitations, you could graft other varieties onto your trees.

OK, thanks for the advice, looks like the branch is going to go, but I'll wait until after the leaves have fallen.

Pity, as it's going to shrink the plant by about 1/3! Just have to hope it's a bit more well-behaved when it decides to send out it's next branch.

Rather than put the branch down into the soil under the tree to root, put a pot there filled with soil, lay the branch down on the soil, cover with a bit more soil, and put a big brick or rock on top of it all.  Come back in a few months, cut the branch off the tree, and a potted plant you have!