I recently took out travel insurance, and phoned loads of companies, the prices varied HUGELY!! And it's also dependent on whether you take insulin, have high BP, Cholesterol, and/or any complications due to diabetes.
Anyway, the best one I found was with Swinton Insurance, which uses Axa. For me, as I'm a 'no to all the above', there was no extra on top for being diabetic, which wasn't the case with all the companies I tried.
The one advertised on the DUK website was one of the dearest I tried!
xx