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Thanks for your replies folks.
It's not the car ride to the vet that's the problem for birds, it's being handled by humans if they are not used to it - it causes them stress and they can die of shock.
Unfortunately I speak from experience here. I used to take in rescued birds (have taken about 100 in total over the years) and have had birds die in my, or the vet's hands, through shock/stress. It is very sad when you think you are doing what's best for the bird. It's a difficult decision to make.
Re this little fellah, he is the last surviving of all the budgies I have rescued. He is at least 14 years old, and could be much older than that - I don't know how old he was when I took him in.
I don't think his regurgitation is due to him wanting to mate (think he's too old) and the worrying thing is that it is clumps he is bringing up, not just a seed or two. I have been on the phone to an avian vet (well, the receptionist - and she is gonna get advice from the avian vet and ring me). So hopefully I will do what's best for him.
Thanks again for your replies folks - much appreciated.x