Husband and wife split up, wife moves out, husband has mortgage, deeds etc in his name, 2 and half years on house is still on market for sale but wife keep going back to the house letting herself in and changing locks. Is this illegal?

International site ? Where is it at, UK, Canada, Iran ? Makes a big difference.

Next is there a formal separation agreement though the court,
Has divorce been filed ?

International site ? where is it at, UK, Canada, Iran ? makes a big difference.

Next is there a formal seperation agreement though the court,?
Has divorce been filed ?

Wales uk
Divorce proceeding have been started, as soon as wife left she started proceedings immediately, its between solicitors, she is now with a new partner and living in new home

Yes it would be illegal for her to change the locks. However, if ownership of the home is disputed in the divorce it may not be illegal for her to enter the home,especially if she has possessions in there.

... wife keep going back to the house letting herself in and changing locks. is this illegal?

Don't know. Why not ask your solicitor?