Nikon 5100 - best buy at sub £400 ?
While smaller cameras are nice in some ways, the size of the D5100 ( actually small and light by DSLR standards ) is not bad. You may be more intimidated by this than you need to be. You soon adapt to the size ( and even bigger systems ).
Also note that the extra size gives you better handling, especially with larger lenses. The balance of the system is better.
Mow I do like the G3 as well, and it's certainly a nice system, but just don't be completely off-put by the D5100 size.