You are right that the conviction is spent now, so there is no need to disclose it in this case. For future reference, as the form had "No" in the box already, that is an assumption that you have no unspent convictions. The information on Unlock's page here says that you need to check assumptions before you take out the policy. If you have an unspent conviction and they don't ask but the policy documents say you don't have any unspent convictions, then you need to tell them because otherwise the insurer can refuse to pay out if you make a claim.

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If you are to punish a man retributively you must injure him. If you are to reform him you must improve him. And men are not improved by injuries. (George Bernard Shaw)

If you were sentenced to a fine on 8 September 2016, then your conviction became spent on 8 September 2017, so it's not an issue.
But even if it was unspent, or if it was unspent when your insurance renewed, don't worry - if you're not asked, you do NOT have to tell them. It is entirely up to them to ask. You do not have to volunteer this information.

For the past 6 or 7 years I've used the same insurance broker for my car insurance.  Usually they will autorenew my policy with the same provider unless I contact them to see if they can beat an alternative price.

On my proposal form there is a box labled "(c) been convicted of or cautioned for any other offence, including offence relating to theft,
fraud or dishonesty?" with "No" entered into it.

Last year at renewal I wasn't asked this question by an insurance adviser as my insurance rolled over as I didn't shop around.

If I'm not directly asked this question by my broker either over the phone or in writing, do I have to inform them that I have an unspent conviction?  Or does it class as being asked as the wording is on the proposal form which they automatically send out, even though they've assumed my answer is still no?

For the record my conviction was for Common Assault, which I recieved a fine of £600, £250 costs and £50 compensation on 8th September 2016.  This will remain unspent for 1 year.