Help - drowning in non-thesis related work from supervisors during PhD
This is an unpleasent, but not uncommon situation. You say you "feel like I am being treated how you may a member of staff", and in fact many supervisors, particularly in the bio-medical world do treat their students exactly this way. For many students are the only mechanism to get any hands to do research. It is a bad thing, but it is common.

Firstly, no one should ever ask you to do work that won't get you an authorship, unless its group maintenance sort of work (organising the seminar rota, or making sure there is always milk in the fridge etc, monitoring everyones disk space usage). I know you say you don't care about this, but you should. Authorship is the currency of academia. It is how we are paid for our work. 

Secondly SOME amount of off-topic work helping others is part of the payment you make to them for training you. When the time comes, you might need help from someone else to achieve something your project requires, but you are not capable of. However, you need to be careful of this becoming too much. In the run of things, your 10-15 hours a week is probably on the edge of what might be considered too much. If you are work 50 hours a week, then thats 20%-30% of your time. Not nothing, but not leaving no time for your own work, probably just starting to interfere. 

Finally, expecting someone to do something immediately, when they are doing it as a favour is plain bad manners. 

You have two approaches. First, if your supervisors are being cold and formal, then be formal back: arrange a set of expectations that both sides are expected to uphold. This doesn't have to be confrontational, it can be approached from a neutral stand point. 

You time scales make it sound like you are in the UK. In which case, you should have an adviser - an academic in the department that is not scientifically related to your project. They are there for exactly this reason. The way this usually works here is that we have meetings with our advisees and if they report this sort of worry we go to their supervisor and say "I ask student X about their day-to-day and the answer made us worry that they are spending insufficient time on their primary project" - makes it sound like the advisor worked it out rather than you complained.