Hello to all. I need some advice. I applied for a MRes program and was unofficially accepted by two supervisors and the head of the department. I am currently waiting to hear from the admission office and get a formal offer from the university. Parallel to that, I was accepted to a taught Masters program and the semester starts next week. I much rather do the research but am afraid to lose the other option. So my question is: how sure can I be at this point that I will get the official offer from the university? If both the supervisors and the head of the department have accepted me and recommended me to receive a formal offer - can something still go wrong?

It is unlikely that something goes wrong. I have done MRes but to be honest, I think taught Master has the advantage of being "normal" and understood by all universities all over the world. The MRes gives you the possibility to publish and might be considered as a practical experience. On the other hand, if you get a distinction in taght Master, you have a high chance of getting a PhD.
To sum up, rethink again (quickly) about pros and cons for MRes vs taugh Master

You need to email the uni and tell them about your dilemma and ask them to speed up the process or confirm to you that it's all finalised.