My new vaoild passport arrived by Post,this cancels the old one with 1 year Thai non O 90 day entry

Away from home at present.

Do I need to visit an immigratin office first to have new passport details confirmed please?

I am sure this has occurred before only along way from local office.

Or can I just arrive at Mae sai checkpoint on last day of my 90 days on non O and present both passports.

Thank you in anticipation

Just take both passports with you until the visa in your old passport is expired.

I renewed my UK passport last month and went to Immigration in Chiang Mai and they transferred various stamps to my new passport including my retirement extention of stay. Will still keep old passport until l renew my extention of stay in October

Depends if you have a visa or extension of stay.

Extension go to your local office.

Visa just do a border run closer to the time and get stamped in on your new passport.

Seems you are not on an extension of stay, so you can just take both passports with you and they will take care of it at the border. That seems to be the normal practise these days.

Thanks guys yes I am on a visa

Yes! Get it transferred over if you plan to go anywhere else but Mae Sai.

A few months ago I tried to cross into Laos with both old and new passports, as others have advised, and they sent me back to the regional immigration office. Thankfully it was only a km from the border crossing - but had we arrived 15 later, the office would have been closed, I would have been stuck on the Thai side (for the weekend!), thus violating my Thai visa, needing to pay an overstay fee, all the while losing my hotel reservation in Vientiane.

So, yes, you could try to simply carry both passports. But depending on where and when you go, you could be in a world of hurt.