Thank you both for your comments.
I'm 67 so I agree it's expected for my kidney function to decline due to age, however it was the fact it went down quite rapidly in 3 months. I've spoken to my gp after my January blood test, appointment booked re the current test on the 16 May.
I have a very basic bupa and spoke to a gp from there about my current test, she said in view of the rapid decline to see a kidney consultant. I think that shocked me I expected her to say oh don't worry it's age related. I was feeling a bit sorry for myself yesterday as I looked at the kidney diet and I'd find it extremely difficult to do that and low carb. Plus the other health condition I have may potentially need a long serious operation if it worsens so I was a bit why me, but hey ho, I feel better today.
Jo, in my experience I'm not in the least surprised by the private GP's reaction - to be clear, not so much because of the decline in your apparent kidney function, but because, again, in my experience, private medics are much more likely to intervene in any given situation, and they are much more acute in their rear covering.
In your shoes, I'd probably be keen to have some further checks and investigations on my kidneys too, so do press for it. If your GP isn't keen to refer you, ask him to explain to you why - IN WRITING. In my experience, GP notes can be scant, to pay them a compliment, and I'd want a decent record of events - just in case.
Whatever transpires, keep us up to date. Fingers crossed for you.