My cat Olly has been showing his third eyelids over the past week, this seems to be the only symptonat the moment. Although he does seem too be slightly depressed.
I tried to take him to the vets today but it was impossible to get him into a catbox, he is a very large cat.
I am borrowing a bigger box tomorrow and I am going to keep it in the house to keep him used to it, and hopefully by Monday I will be able to get him there!
Please help I'm quite worried

Your best bet is to not worry- I don't believe this is a serious symptom, you could compare it to several other symptoms that would cause more alarm than this. I would just take him to the vet, keep him comfortable in the meantime, and don't be anxious. The more anxious you are, the more anxious he will be- so relax, and be open to what the vet has to say.