I seem to be taking alot longer than others around me to write my thesis. I have thesis plan that has been agreed with my supervisor - but its the depth than I'm concerned about. I seem to go into alot more depth on stuff than others around me, but can't quite figure out how to do less without missing out important stuff.

I have had a look at my colleagues thesis - and i really don't think they are very good. They don't explain anything - but yet one just had his viva and passed with tiny minor corrections

Should I follow their lead and just get the thing done, or should I continue doing it my (v.slow) way? Its a science PhD based on experiments by the way.

It's your thesis and will be your thesis for the rest of your life. If I was you I'd do it exactly how you want to until someone starts demanding otherwise. If money/time is not a major isuue, relax and do it thoroughly and comprehensively. This way, when you go into your viva you'll be confident. If you try to skimp like the others you'll be shitting yourself come the day.

I reckon that you might be misunderstanding slightly what a PhD actually involves. There's obviously the risk of doing less than expected but also the risk of doing more than expected (which seems to be your case). I would suggest you to think about your limitations: is your funding running out soon, can you support yourself for 4, 5, 6 or the number of years you need to do the thesis the way you would like, is there a particular date you would like to finish your PhD by, would it be harmful for your future career to have invested 6 years in your PhD instead of 3 (employers won't probably read your thesis and won't know that it took so long because you wanted it to be perfect)?
If you are fortunate enough not to have any financial/familiar limitations, just take your time and do the PhD your way.
If you do have limitations, concentrate in achieving a satisfactory standard for your thesis and bear in mind that you can always expand your research later through a postdoc.

Make sure you are clear on the difference between depth and detail.

Focus on who you are writing for i.e. the examiners, people who are knowledgable in the field. You don't have to explain it so that it is comprehensible to an undergrad.

your original contribution will obviously need clear explanation. But it is not like a taught course where you need to explain well-established ideas to prove you understand them. Be clear on who you are explaining things for - if it is for your benefit or to prove you know it, rather than to make new knowledge clear for the examiners, then it is not strictly necessary.

Also bear in mind you word limit, and that to endear yourself to examiners, the shorter the better.

I haven't been to the writing up stage yet but I would say that you should write up in your own way and your own style. Going into detail is fine for your own work explaining the decisions you made and why you made them, but I would say you shouldn't be going into the same level of detail for other peoples work (unless these are the building blocks your work is built on).

I would speak to your supervisors and see if they can give you some pointers on some areas that you are going into too much detail (if in fact you are)