I am moving soon to Geneva, and have had a salary simulation from my new employers that shows 'GE' canton tax, along with the unfathomable list of insurances and so on, but I have no idea as so the sort of range of commune tax that will apply - is this a small or significant amount? Any approximations welcome. Thanks.

The commune tax is about one third of your whole tax burden. 1/3 State, 1/3 Canton and 1/3 Commune.

If I understand your question correctly.

Um... does this mean then that if I have been given a figure for my income tax and canton tax combined, that I should factor in another 50% of that for going to the commune?

If you're taxed at source, it's all bulked in. If you're not, you can simulate the tax rates on homegate.ch.