Peggyundercrackers you might find it affects your insurance claims anyway 
Somebody drove into dh in a car park (he couldn't turn his head so used to check it was clear before getting into the car and hope it was still the case when he reversed out of his space 
). He admitted full liability and although we told our insurers, his insurers dealt with all of it. However our premiums went up, despite us being completely not at fault. Apparently it's because having been involved in an accident we are now at higher risk of being in another one. And that we still get the same no-claims discount as before (ie the same percentage) but as we are now higher risk, the base price before the discount is applied is higher to reflect this risk.
I discovered this because when I was getting quotes the year after it happened I didn't mention it to my insurers and they came back and said you had a claim, I said I didn't, thinking they meant we had claimed on our own insurance, we were at no fault and all other person's fault and insurance, at which point they said the above. I genuinely wasn't trying to diddle them, it hadn't occurred to me that this situation would be counted as a strike against us as well as the bloke that actually caused the accident, plus the wording wasn't very clear given the circumstances. They also pointed out that should we have needed to claim on our insurance later on, they would have been able to turn down our claim on the basis of not having told them everything for them to give us an accurate quote.
These days I've noticed that the wording is often much clearer and more explicit about making any claim regardless of whether or not you involved your own insurers. And I always make sure that I mention it when getting quotes. I've also discovered that you have to keep mentioning it for at least 5 years and they want quite detailed info that it hadn't occurred to me to keep - actual date of the accident, how much claim was for etc etc - so it's worth writing it all down and keeping it somewhere handy so you can pull it up each year rather than try to dredge it out of your memory and discover you have a detail wrong. The one year I couldn't find the details I found it was easier to get the approximate quotes online and then speak to someone as on the phone you can say that it was about this time or cost about that much or that you can't remember - and they can deal with it, whereas online systems are very black and white and can't cope with that sort of fuzzy info so you can inadvertantly end up giving wrong info, again which might invalidate future claims.
Definitely not worth hiding it from your insurers as you might end up with bad knock on effects!