Does anyone know what's required when doing the above.
Can't find much info on DVLA site except for vehicles inported
into the UK. Now I'm preeettty sure NI is still actually part
of the UK so is it simply a V5 correction as per normal ownership
transfer ?

Nige

The Government has set up DVLNI to act in Northern Ireland. Not sure if it is actually a subsidiary agency of DVLA or separate, but DVLA proper only covers Great Britain. http://www.dvlni.gov.uk

Technically you are right, though - you can't "import" from NI because it's already part of the UK. However, you have to notify DVLA and re-register it. In practice I think you keep the same registration.

You could try something radical and phone them!

Happy Easter