I’ll be arriving on a 90 days non o imm visa and want to know if I can extend this for a further 30 days at immigration. I do not need any longer, just a 30 days extension. 

You cannot extend a non O by 30 days.

If married to a Thai you can obtain a 60 day extension to visit wife.

You could do a border bounce for visa exempt entry currently 45 day stamp

With a Thai spouse or child, you could get a 60-day extension. Otherwise, depending on your nationality, your best option might be a border bounce (leaving and re-entering Thailand) for, most likely a visa exempt entry (45 days) or a visa on arrival (30 days).