Thank you for all you responses,

He's on a course of Ceftizidime antibiotics which consists of 7 injections every 3 days, he had his 5th yesterday. The snake (Patches) is only 3 months old.

To be honest no I don't know the difference I would presume the mucus is thicker but I am trying to do a bit of research on this.

I spoke to my vet today he didn't seem overly concerned and has given me some more antibiotics and talked about a few other options like testing for other viruses and x-rays etc. He did say if we wanted to try and remove to the mucus to do so but I don't really feel comfortable poking around in his mouth as it would stress him out more.

The symptoms are:
Constant dribbling but this has got better
Head raising
Yawning/sitting with his mouth open
Wheezing/sneezing-I know they don't actually sneeze but best way of describing the sound
Off food he hasn't eaten since 18th June (we keep a book)

We have noticed that Patches doesn't really seem all that bothered about it (he's staring at us now) although of course he has been a lot quieter and when he does come out of his hide it is not for long. We have down sized his food to help him swallow easier but been trying to feed twice a week so he still gets what he usually would.