PhD supervisor making me do a LOT of non PhD work 2nd year PhD student going into 3rd and final year in September, in the UK. 

My PhD supervisor is a brilliant researcher, and for the most part I enjoy working with her. I do have a second supervisor but she is essentially useless. 

My project is pretty hectic, I enjoy working on different components at once. That said, I have a defined research question and thesis chapters. 

However, recently my supervisor has tasked me with helping on lab work for a project which needs to be completed for a grant application. This project has nothing to do with my PhD (other than I will be using the same method). It involves about 10-12 hours of hands on time per week. This is a significant amount of time out of my work which is intense enough without this. I mean I would frequently work from 7-7 on it alone. 

I have been told I can only be paid for the weekend work (which consists of about 1 hour of the weekly 10), but this to me seems dodgy. The work has nothing to do with my PhD and is directly impacting my ability to get stuff done. 

The fact is I'm scared of saying any of this to my supervisor, as at the end of the day I am planning to do a post with her and I feel like if you present as inflexible it could hurt me down the line. However I do feel a bit taken advantage of, and at the end of the day considering that doing this work for a few months is only prolonging my thesis submission. 

Just wondering if anyone else has experienced anything similar or has any advice?
Although there may be a number of people here who are quick to claim that no PhD advisor would ever take advantage of the cheap and free labor that PhD students can be coerced into (by means of using degree and thesis requirements as a figurative cudgel), I think that it is somewhat common that PhD advisors make use of such free labor. This should not be. 

This being said, there are a few things to consider:


The grant application you are assisting in might be a direct means of funding your future research. That is the "pay off" for you. Nothing happens in academia for free. Make sure that you are being compensated, but also keep in mind that not all compensation is directly and immediately monetary. 
A few months of weekend work is sometimes part of "paying your dues" in academia and can open up further opportunities for you. (Such as a future post-doc with this professor). 
Address your concerns as to timeline with your supervisor. If she dismisses your thoughts entirely, you may want to consider if this is really a professor you want to do a post-doc with.