TB48 idle issue

Hi all
Strange one I hope someone can help with…

Occasionally when I start she will idle at around 1500rpm. Revving makes no difference and the only way to stop it is to switch off and start her again. As I say, this only happens occasionally. Any ideas?

Thanks

Nick

This issue has now progressed and she idles at around 1700rpm constantly and seems lacking in power. No diagnostic codes are showing. I did have the plugs changed a few months ago. Anyone have any ideas where I should start looking?
Thanks