Hi I am worried that I am not doing enough work! I'm at uni from 8am till 4pm, either reading or in the lab and occasionally I read at weekends. What does everyone else think, is that enough?

Well, what subject are u doing your phD in? What year are you in? Are you getting results? Do you work from 8 to 4 or do you also hang around and go for a tea break?
What does your supervisor say? Has he/she comented on your working houres?

if you want to be the best in your area, be sure. nothing is enough...

I am in my first year and I have just started expts that will go towards my PhD. I work the whole day except lunch and the odd look at this forum!! My supervisor hasn't said anything but I don't see her very much anyway.

It's actually up to each individual. I work from between 6am and 7am until about 2-3pm, often without any break. I find this works best for me as it means I can get on with things such as PCR's and have them all finished before the rest of the lab come in (often as late as 11am!) This also allows me to have some spare time to do my own things (which I need desperately as I'm renovating a house too!) And I'm at the end of my first year too.