Does anyone know why witnesses in relation to conversion 'Retirement' to 'Marriage' have to go to Immigration when the 4 pics plus house book and ID card details already supplied.

At least in Jomtien, witnesses are not required at immigration anymore,  since immigration now does house visits not only on the first extension, but also on renewals. This my experience from last November 

Not all offices want witnesses. They are needed to confirm you are living together as husband and wife.

Same reason they do home visits.

It is to confirm this line in clause 2.18 of the police order. "(3) In the case of spouse, the relationship must be de jure and de facto"

Thank you UJ for your reply.    As always you at least can be relied upon for factual and sensible information.  Much appreciated! 

came to our house today ,nice guys ,had a laugh and a chat ,our neighbour came in showed her paperwork ,we all had our picture taken outside the house ,no problems .