My girlfriend bought me a group of 4 Japanese Maple bonsai trees, aged 8 - 10 years, from Dobbies 3 weeks ago. As I work overseas it was left in her care for those three weks, in which she kept it indoors, albeit adequetely watered. I received a panicky email on thursday advising me that the leaves on 3 of the 4 were wilting badly and asking what she should do. I told her to put it outside immedietely, which she did, and I checked it this morning upon my return - but its looking a bit knackered. I only ever kept indoor bonsai before, but thought that the acer was fairly hardy and would be okay if left ouside. We are away on holiday for 2 weeks shortly, what shoudl i do to prevent the trees from dying? Leave it outside and hope for the best???

All advice gratefully received...

Sounds like your girlfriend left your tree's to get too dry before watering!!! Your tree's are in distress! People often will notice their house plants are dry because they look wilted, so they give them some water and they just perk back up. But with Bonsai that level of wilting is likely to be too late. A wilted Bonsai or one that is dropping its leaves spells real trouble. It will not recover the same way a houseplant does.
The Bonsai has a bare minimum of soil, and once it has been allowed to get bone dry, it is very easy for the tree to die or be severely damaged. Therefore, you really have to check the tree often.
One of the main problems with house plants arises from over watering, but with Bonsai, one of the most immediate problems is under watering. So the first thing you need to know is if the tree is wet or dry.
Place your tree outside where it can get morning sun ONLY.Move to shade in the afternoon,protecting from wind. Keep the tree MOIST not soaked,(you can actually drown a bonsai!)taking care not to chill the roots. Hopefully,and I do say hopefully with caution, it will recover in time. Do not feed the tree whilst it is in distress.

Like most maples, a Japanese Maple is somewhat adaptable but grows best in deep, well-drained, fertile soils.
It grows well as an understory tree, tolerating fairly dense shade. I have many Japanese Maples and i have more or less left them to do wat they like. When i have a new one i make sure it is well watered and give it some fertiliser for the first few months of its transplant. The only care i give the older ones is a little prunning at the end of every growing season. I hope it stays alive, they are my favourite tree.

Without photos of the leaves, it is a bit hard to tell what is wrong. They should definitely be outside regardless. Keep them in a ventilated area with mild filtered sunlight to give them their best chance of recovery.
However, If you are both on holiday for two weeks, without water they will die, so I guess any advice I give will be pointless anyway.