OP
Above all you need to remain clinically calm and absolutely in charge of your emotions at all times when you are dealing with SS. Cooperate fully even if they ask outrageous things. Be absolutely calm at all times.
Secondly you need to immediately contact your MP. This really needs to be done today. You explain everything to her/him and you ask directly for their intervention. They need to represent you, they need to demand for an early court hearing if that is what SS intend to do.
Op, take your phone and the message from the nursery at 11am re: the calpol and ask them to consider this immediately. It is evidence that something was clearly amiss IN the nursery way before they alerted you to the lump in the afternoon.
Given the lump started to appear in the late afternoon then surely it must be almost medically impossible that you to have had anything to do with it. Go to your GP and ask him/her what are the chances of this if happening. Get it in writing if it supports your case. Usually when a child or adult sustains an injury the lump appears straight away.
Please try and gather witness statements from the workers that took your son in the morning, get them to confirm IN WRITING that he was fine and in good health when you dropped him off.
You need health visitor reports that are positive about your parenting, other friends, family anyone you know that can write a character reference for you.
If you don't have the money to get the best lawyer, then crowd fund today and raise the money. I think most people would happily help you get your child back.
Speak to your Aunt and Uncle and ensure they are on board to continue, do whatever you need to, to keep them happy.
I am so sorry this has happened to you, but if you stay calm and together the truth will emerge.
In my view, the most likely suspect in all of this looking from a purely neutral stance is the nursery. They were with him for so long before this was even raised.
Your son's removal is a preventative measure until the facts are confirmed, the SS are in no way saying this is your fault or you are responsible. It is purely a safely measure - keep this in mind.
If you get into a desperate situation, call every newspaper in the country and ask for their support. As much as we hate the DM they are often very good at campaigns like this.
You will come through this. 