Good thread - its interesting to know the freelance rate for 3D artists (for the future).
I think (from occasionally doing web freelance work) you can get stuck in the original position of charging the same low rate as when you start out. It can almost be a barrier to up your prices as you think you'll scare people and business away.

Maybe a graded price increase could be a way forward. For all new work then set the fee at £35-£40 but for your current close clientbase (the few you work well with) you could up it by £5-£10 every year till it matches your new rate. In a way, if it ever comes into question that you're charging different rates then you can use it favourably with your close clients (you scratch my back, I'll scratch yours). Also as the other guys have said, if questions come up then say its to upgrade systems which will ultimately benefit the client (faster rendering, more tools for effects they want etc).
If you give people enough warning about charge increases then they will generally accept it and factor it in to any future work (or forget about it till payday, argue a little, realise they had already been informed and then pay).

Si