I may have misunderstood the original question, but I thought the main concern was about whether it would be disclosed to the employer.

It would be highly unusual if such questioning during a trial would be reported at all given the shortage of court reporters. Even Court News only concentrate on the main findings unless it is a notorious case.

And as long as the prosecution are aware, if the previous conviction is irrelevant to the case in hand it would be challenged by their barrister. I may have this wrong but during case management, the prosecution and defence have to lay out their evidence. If the competency of a witness was going to be an issue, it would have to be revealed in advance. TV "surprise" witnesses are not a normal happening during a trial.

Whichever side calls you, the law says your record should be disclosed to both sides. That doesn't mean it will be raised during your testimony, but it could be if they're desperate to discredit your evidence and they think it might sway the jury. As your conviction has been spent for some years, it might well not be mentioned, but it's probably worth making sure that the side calling you knows about your record. If they don't like it, they may even decide not to call you as a witness....

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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)

These are 2 very helpful and re-assuring replies - many thanks
The conviction was prior to my current role and was spent 4 years ago, so its good to know I could ' take them on' if they divulged it
Thanks again

What the CPS will want to check is that you do not have a prior conviction for something related to the case (e.g. a fraud conviction) - they do not have to disclose it but want to be aware in case it is information that the defence could obtain (old press reports / google ) and try to use it to discredit you.

In my experience (as a prosecution witness for the CPS in a case), they aren't that well organised and no background checks were done.

You don't say whether you acquired the conviction during your current employment but if your conviction is spent and your job doesn't require an enhanced check, then the police would be in breach of the Human Rights Act in revealing it to your employer as they have no legal reason to be told about it. 

One of the worst aspects of having a conviction is going through all the possible combinations of what "could" happen if people find out. I personally think that it is prudent to think of how you'd respond, but assuming that the worst possible case is definitely going to happen is not a good way to live. Yes, think about what you might say, but only worry about it if it happens.

I have been called as a witness in an HMRC smuggling investigation.  Neither me nor my employer is implicated, I'm just called to explain my company's position and explain what happened with paperwork, but they have said they need to do a witness check on me before they submit the case to the CPS.  My conviction is spent, and all they asked for is my name, DOB and address, but I am concerned my conviction will come out and my employer could be told ...

Does anyone have experience of this please ?  DBS website asks for a proof of id and they haven't so is it a simpler check of some sort ?

Finally any advice on how I respond if they find my spend conviction and want to disclose to my employer ? I'm thinking a) they can't and b) I argue its irrelevant ?
Any thoughts gratefully received