Hi
@Gwyneth Pocock and welcome to the forum.
I think you most definitely should declare, because should anything happen to you that could conceivably be linked back to your diabetes, the insurance company can legally wriggle out of responsibility and invalidate your cover.
I'm no expert, but just happened to have spent much of the afternoon investigating and getting travel insurance, so it's fresh in my mind.
You'll also need to declare your Covid infection if it was within the past 12 months, assuming they're all doing the same medical screening as the company I am using.
Fortunately for me. I'm Type 2 and not insulin dependant, and have so far escaped Covid, so I don't think my premium was too inflated.
I hope this helps - but I'm sure some insulin-dependant posters will pitch in with their experience.
Good luck with it and enjoy your travel
Nick