I wasn’t with Aviva, but mine was no trouble at all. Very simple claim form. They wanted all my GP records, which I had to give consent to. They also asked permission to contact my neuro, but never did (I assume because all his findings had been covered in letters to my doctor anyway, so they must have thought it was all quite clear enough).
You are right to be thinking about it sooner rather than later, because there’s often quite a tight time-limit after diagnosis to get the claim in (mine was just three months).
Possible sticking points are if you’d ever been to the doctor with any neuro-type problem prior to taking out the insurance, but not disclosed it. They could treat this as something that would invalidate the policy, as they could argue they would not have covered you, or would at least have charged higher premiums, had they been aware of any factors that might have pointed to neurological disease.
There is nothing you can do about this retrospectively. If there are things in your medical history which, with hindsight, could have been early hints, you’ll just have to hope they’re not significant enough that the insurers believe you - or your doctor - had cause to suspect anything serious. As long as they only emerged AFTER you took the insurance, you’ll be OK anyway.
The other fly in the ointment is that some policies state you must have had at least six months continuing disability. I wasn’t (and still am not) visibly disabled, so I was concerned I wouldn’t meet this part. In practice, though, it wasn’t a problem, as they seemed satisfied with very slight evidence of disability - my physio had written in the notes that I had a weak ankle - something I wasn’t even really aware of. But it seemed sufficient. I suppose the fact I needed a physio at all must have been some evidence I wasn’t fine, but I didn’t have to prove I needed a stick, or anything like that - which was just as well, as I didn’t.
I also didn’t have to prove I couldn’t work, because it was critical illness, not income protection, so the key thing was having a confirmed diagnosis, not how bad it was. It does sound a bit funny having a “not bad” critical illness, doesn’t it? But it’s just insurance industry terminology. “Critical” does not mean you will die of it.