Seatheight issue

@big mick - I did read your question and nowhere does it state "My seat height is right". It states " the seat feels right if anything a bit high but after say half an hour it feels like it could do with going a bit higher" which, to me, implies incorrect saddle adjustment. If you had stated "My seat height is right" in the first place I wouldn't have bothered posting a suggestion. Good luck with your issue.

See this GNC video on tips for getting your saddle (not seat) position correctly.

https://www.globalcyclingnetwork.com/video/how-to-set-your-saddle-height...

I had this the other day - saddle felt so low, I was checking to see if it had slipped. Felt fine ten minutes later.

Or your feelings are entirely subjective and don't reflect any actual change in the effective seat height. When you move about on the saddle your effective seat height (distance from pedal spindle to point of butt contact) changes by as much as 12 mm, so in spite of what many might say, your seat height (+/-5) mm is not a critical measurement. 

Slightly more in-depth, your perception of saddle height comes from your proprioceptors, nerves which 'sense' the length and tension of your muscles and tell your brain where in space your limbs are. As your muscles warm up and become more supple then the proprioceptors will send slightly different messages to your brain, altering your perception of where your feet are relative to your butt, (or vice-versa)

Try it, but make some measurements or mark so you can go back if it turns out not to be comfortable.