Just a quick update - I feel this thread will be of some use to anyone who sees the same symptoms in their Degus, hopefully then once a vet is told they can receive treatment quicker than my two did.
I have just returned from the vet with Sid, he's doing much better and has put on 20g since last week getting him up to a much healthier 190g. His feet are still a bit sore and he squeaked a little on examination so will need to continue with some pain meds the vet prescribed. I've also got to keep him on the Baytril for at least another 2 weeks.
This visit the vet explained that my boys were suffering from a viral infection of the lungs.
Unfortunately she said this is likely to have been something that they had in them when I purchased them (which explains their brother being taken off due to an eye infection when I bought my two). This is usually fatal however whilst sadly Diego passed away Sids health is improving vastly so he's a lucky and strong boy!
The sad part of the visit was that the vet told me that this is something rather like the coldsore virus which may reoccur (I really hope not!) Due to this I've been told that it is contagious so I will be unable to risk getting Sid a cagemate/s as there is a chance they could contract the virus and go through the same ordeal.
I feel so sad that poor little Sid is going to have a lonely life, just feel miserable that he'll never have another degu to snuggle up in his hammock with.
Now it's time to keep his little spirits up and give him plenty to play with and lots of human social interaction. His life may not be as fulfilling as it would if he were to have a buddy but I certainly hope that I can stop him from getting lonely by giving him lots of attention.
Thanks to everyone who contributed to my thread, I honestly think I would have gone mad without speaking to people who understand why I care so much for these little animals.