I split up with my wife almost a year ago, I have since met somebody else and I no she is the one I want to be with.. my wife is now saying if the children see my new partner she will stop me from seeing them. Is she allowed to do this?

You need to get a lawyer and get your visitation defined in court.
It is the only way you will have that guarantees you time with your kids.

When the problems between you and the ex arise, and they will, the kids are ususlly the first pieces played with total disregard to their emotional well being, security, and love for BOTH parents.

Get a second job, sell your car and take the bus , but come up with money for an attorney and get it on court documents

I have been there, and I have seen many others who have been there.
Attorney, now, go.

Is the separation court ratified? Was there any visitation ordered. Your ex will probably not be able to get a court to prevent you from seeing your children because you have moved on. However, as we have often seen here a custodial parent can make it difficult to enforce a visitation order.

She will have to follow the child visit agreement that you got when you left your wife.

If you do not have an agreement, why ? That is what you need to do

You do keep saying "wife" so you get a divorce, file for visits with the kids, and your chlld support.

Unless there is a court order for support, yes she can stop you since they are living with her.