I can't really tell what it is could you clear away some soil around it so that we can see it in relation to the roots and trunk, and take another picture or two.

As you do it keep an eye open for stringy reddish/ black threads and have sniff to see if you can detect a mushroom like smell.

I hope you don't but I want to eliminate the possibility of "honey fungus"

Here is an article on the fungus and it's symptoms read this before you do what I asked then you too will know what to look out for.
http://www.thegardenersalmanac.co.uk/Data/Honey%20fungus/Honey%20Fungus.htm

Sorry dont know the answer to your problem, but hope everything will be ok. Thought people interested in plum trees may have an idea on my tree, it is a two year old Marjorie seedling, planted in the garden, very healthy, last year had tons of fruit, this year nothing, no blossom at all....any ideas? Only thing I can think is that I gave it a light pruning after fruiting last year to keep good size. It is sprouting new growth and does look healthy.

Whatever it is, it won't be honey fungus at this time of year - the toadstools for that don't appear until September time.
I've got a horrible feeling it might be Crown Gall though - this is caused by a bacterium, and can be a serious issue on woody plants. If it is that, you will need to remove the tree at some point, as it gets worse, and not grow any woody based plant in the same area for at least year, so soft stemmed plants only, such as herbaceous perennials. Although, if its on your patio, maybe its in a pot? If so, don't reuse the compost, and thoroughly clean the container before re using.

Crown Gall. Nice one, Bamboo. That's a new one on me. I shall make a note of that one to keep for future reference.

Thank god for the RHS Pests and Diseases book, Jimmytheone... there's an aerial form of Crown Gall too... fortunately, you don't see either that often.