Can an employer insist on CRB clearance several years into an employment contract.?

The work is that of giving advice to people who want to start a business. Recently management have taken the view that CRB checks should apply because of the remote case of advicing to someone with mental problems who is considered vulnarable. The initial employment contract did not include it as a condition.

Hmmmm, this is exactly the kind of contractual change that should be negotiated with a union - and it is a contractual change.
There needs to be clarification about what type of offences would be deemed to be a problem.
What would happen if an previous offence crops up from years ago?
Are they going to dismiss that person? (Given that they were recruited and have been allowed to operate up to this point without a check.)
Are they going to require them to interview people in a public area?
If these things have not been discussed with management then it is another example of a management team rushing into making a decision feeling they are doing the right thing without thinking about it thoroughly first and without involving their staff. (Clearly you have not been consulted nor informed of the ramifications.) Things like this ought to be clarified before such changes are introduced.

I believe the point that AA makes below is factually incorrect.
If it were a LEGAL requirement - why was this not done when your were initially employed? This is nonsense. As you have clearly stated in your question this is a management decision to cover themselves in the event of anything happening. (If they had failed to have CRB checks done they are probably concerned that they might be sued for negligence - but it is not a legal requirement.)
If the outcome of a CRB check is to have an impact upon your continued employment, when it has not done so before, then that to me is a contractual change, the implementation of which should have been discussed with a union. It seems clear to me that your company has failed to take steps to do this and to reassure it's employees, which is bad management.

Yes. And they can ask you to complete a CRB Disclosure Application at regular intervals.

Yes, why not? Its in the public interest especially if you are potentially working with vulnerable people. I think its a wise decision to make, especially in this day and age.

I know that people will say here, "What are you hiding what is the problem?" but yes it is irritating.
I was involved with a volutary organisation which insisted that new volunteers got a CRB grey form. I did not need one but I felt so uncomfortable telling other people to get one that I applied for one. The advantage to you is that you can use this for other employment the form is yours and it is reccomended that it is renewed every three years. I would say that very minor offences wil not trouble you but if you are convicted of violence or sexual misbehaviour you may have a problem

have you been promoted or gone to a diff dept if so they can

Its not a contractual change its a legal change applicable to anyone that works with vulnerable groups and individuals. Your employer has the right to ask that you carry one out, most companies are now doing them. The one that they are requesting from you is enhanced crb because you work with people. The request for people that work in any fields with data protection issues is now a standard. The difference between the two is how much details of your history they go into such as police databases and so on.

If there is a problem with your crb or your expected outcome ideally discuss it with human resources, trust me they have heard it all. When i was interviewing individuals, i heard people that had been done for assault by the police but they had thrown flour at a racist and i had a guy who had been done for being cheeky to the police and fighting when they were kids and so on. What I'm trying to say is that there are many people that have records that really have been criminalised for no reason and human resources and people know that because there are so many.

Good luck whatever your own situation is.

The rights and responsibilities of employees and employers under employment contracts vary by state, and where state law does not specify, depend on the terms of the agreement signed at the time of employment, if any. Generally, however, any agreement that has been in existence for years cannot arbitrarily be changed by either party. You and your employer are bound by the terms in effect at the time when you signed up, both State law and contract terms, not the terms under which new employees are being signed up now. It is also unlikely to be legal for the employer to terminate you for the sole reason that they wish to change your employment agreement against your will. In order to compel you to pass such a check now as a condition of continued employment, the old agreement must allow the employer to change it unilaterally, or State law must allow them to do so. Review your employment agreement and the statutes in your state.