Sadly mate, the only way to get them to reconsider is to get your HbA1c down (I would say as a rule of thumb get it under 8.0 for starters, but obviously the lower the better). You're pretty much certain to get charged extra for life insurance if you're diabetic, and you will be hard pushed to find any company to offer you any terms at all for critical illness cover or permanent health insurance. The risk is deemed too high (shit I know)! Otherwise, its the usual advice, stop smoking, limit your alcohol intake, keep your weight, blood pressure and cholesterol down, and work on your diabetic control (keeping your blood sugar and HbA1c low, and of course not having any hypoglycaemic attacks helps).
You were just unfortunate that you were in the middle of applying for cover when you were diagnosed, as most companies will postpone cover in those cases. If you had been diagnosed 6 months ago, they would have offered you cover, but it would have been a hefty premium.
I agree with what tycoon said though, I don't think they can insist on you having life cover to cover a mortgage anymore (although obviously it is recommended)!
Hope this helps in some way, and I hope your house purchase still goes through.
Best of luck to you and your missus mate.