Mark, I don’t have a mic at the moment, the G3 only has onboard and reviews talk of external noise being an issue. The 600D will take an external mic so if there are any sound issues I can over come this. Until you get the kit and use it you just don’t know how it will perform for you and if you will be happy with it.
While that's true, if you're not videoing something in which sound is an important aspect--e.g. kid's piano recital--I think it's a mistake to prioritize potential audio issues over actual video issues.
... the 50mm f1.8 II ... from what I have read this lens is good for video as well.
Also true, but in addition to the lack of IS that Rorschach noted, also be advised that the non-USM AF is rather noisy, and that it can be difficult to "pull focus" with the thin, loose and "short-throw" ring. Also note that with the 600D's 1.6 crop factor, 50mm (80mm equiv.) may be a little long in tight quarters.
The FZ150 looks great it’s not that much cheaper than the G3 £390 vr £420 with £40 off, I need to do comparisons of the stills quality. I would assume that the G3 and 600D would be a lot better than the FZ150 but I could be wrong!
In good light (i.e. broad daylight), the cameras will probably be very close. But as the light decreases, the larger sensors of the G3 and 600D will have an advantage, especially if they are used with a fast/bright/large aperture lens.
I assume you are saying that the speed/brightness is an issue with the kit lenses and I would be able to overcome this with another lens?
Yup. Both kit lenses are f3.5-5.6; whereas, at f1.7 and f1.8, both of the primes I mentioned are two stops faster than the kit lenses at their fastest aperture. And remember that each stop doubles (or halves, if you want to look at it the other way) the amount of light. So a difference of two stops is four times the light. (Quick and dirty demonstration: light 4 candles, note how bright it is, then blow 3 of them out.)
I need to compromise something, I just need to work out what!
Well, in addition to prioritizing video over audio, I would also suggest you look at which kinds of pictures you will be taking more of in the near future. (read: before you can save up for additional lenses). i.e. over the holidays, will you be taking more indoor pics of the kids and family gatherings or daylight winter scenes or wildlife or ... ?
HTH - Mark