At the risk of beating a dead horse, I'm posting one more pic. Only because I see additional crack forming in the bark. I wonder if its because the tree is exposed to morning sun as well as afternoon sun? Is it possible the trunk being directly exposed to sun in the morning (as opposed to afternoon when its shaded by leaves) is causing this to happen? I can see atlease two additional splits forming in bark and they are all on the side of trunk facing morning sun. That being said, I just realized I haven't really looked at the other side of the tree............. Uploaded with ImageShack.us

It is probably not too large to transplant if you are really strong and have a couple of people to help. The best time is when the tree is dormant, around February. You can go ahead and dig outside the the drip line of the tree and cut down to break up the soil and let it stay this way until you are ready to move. Obviously, you want to get as much of the rootball as possible. (Also, have the "new" location for the tree ready before you dig up the bloodgood) Yes, it can take fun sun. I am in NC and have no problems with JM Bloodgood being in full sun. Perhaps Shaina Japanese Maple would be a good choice for the 3-4 foot tree in full sun> Hope this helps.

Japanese maple foliage is extremely sensitive and spraying it with anything other than plain water can create a phytotoxic reaction. FWIW, with the vast majority of woody plants - like your little bonsai - powdery mildew is purely a cosmetic issue and nothing that will seriously harm the tree. I agree 100% with cearbhaill - just let it be. Misting is completely unnecessary. Water as required at the base, thoroughly wetting the pot and potting medium. The tree should releaf. JM's can lose foliage for a variety of reasons and still produce a new flush of foliage. I'd keep it out of direct sun at all times until it has responded to a more normal growth patterns. And generally I would advise against any fertilizer but containerized or bonsai'd trees DO require supplemental fertilization. I'd wait until the plant has releafed and then use either a single application of slow release (like Osmocote) or a dilute liquid every 2-3 weeks.

Having grown, sold and planted Japanese maples commercially, professionally and personally in the PNW for years and being familiar with your UK climate (British parents - many visits), I'm going to cordially disagree with firefighter :-) Parts of the PNW experience summer drought conditions that do require some specialized watering concerns to young JM's -- but far less than to those sufficiently established. And sun scorch is not a given regardless of the amount of sunlight received, provided adequate soil moisture is maintained. These plants are just a lot tougher and more adaptable than many give them credit for. And full sun in Dorset, UK at that latitude is just not comparable to full sun at other, more southerly locations, including the Puget Sound area. These plants are able to grow in containers, in full sun, on gravel or asphalt nursery beds here without problems or leaf scorch provided they are kept sufficiently hydrated. Or at least my multiple decades of experience with them has proved this to be the case. btw, this included experiencing record high summer temperatures this past July.