I'm now about 18 months into my PhD and I currently looking around at mates that are at/coming to the end of their PhD's who are frantically writing up.

I feel that at this point I have enough of the background and enough results to think about starting to write up a couple of chapters. But friends in the department are saying that I shouldn't even think about writing up for another 6 months. I was just wondering when you guys first started to think about writing up and if you have any advice? The thought of spending a solid 5-6months just writing up will kill me

I can't help on this, I'm only a year in, but just wanted to offer you the best of luck with your writing when you do start!

I would say start to write up as soon as you can and if you have a couple of 'easy' chapters to start with then that's even better! It's very motivating to be able to tick off completed chapters on your list. Writing up ALWAYS takes longer than you think it will so don't delay, start today!! Good luck

I've started a few weeks ago and already my supervisor wants me to start writing; he says I may not keep all I write now, but its better to start when you can and get things moving

I haven't completed all my research (social science), but I've been told that it's best not to think about your PhD as split between research and writing up because as you write you will identify new areas or new arguments that need to be addressed, which may involve going back and re-assessing your research. Not quite the same with the sciences, but I can still see where it would help you to start writing if you feel you can

I know that the writing up will take me ages, just to sit down and write a small report or paper takes me ages, so something the size of a thesis I feel if I could get bits of it done early then it would be so much better

If you know what you want to say then go for it - TBH even if you don't know what you want to say I find writing helps focuses my thoughts