When I was diagnosed in January, I called my car insurers (Saga) to tell them but they didn't want to know!
A couple of weeks ago my insurance was up for renewal and I called them again to ask if having a three-year licence would make a difference to the premium - it didn't.
Saga is for the over-50s so I suppose they're used to older people with chronic conditions.
When I was getting insurance quotes online, a couple of comparison websites asked if I had a notifiable illness, but I wasn't asked for details, just whether I'd notified the DVLA and whether I had a three-year licence. I'm not sure if it had an effect on the price quoted - if it did, it was very small.
My advice is to be honest - otherwise you might get a nasty surprise if you try and claim. Insurance companies don't exist to pay us money when something bad happens, they're businesses and will do everything they can NOT to pay out - that's why there's so much small print!