Hi Ralph, of course I remember you, I hope you are keeping well.
As I understand it (and I'm learning more about this all the time) the IVA has replaced the SVA test and is aimed at individual or small batch new cars etc, the BIVA (BasicIVA) is a less stringent test aimed at modified vehicles. The DVLA rules on what needs a test are also pretty much the same as before (from the DVLA website):
Allocating a vehicle registration mark
The vehicle must score eight or more points to retain the original registration mark. If less than eight points are scored or a second-hand or modified chassis or altered monocoque bodyshell is used an Individual Vehicle Approval (IVA), enhanced single vehicle approval (ESVA), single vehicle approval (SVA) or motorcycle single vehicle approval (MSVA) certificate will be required to register the vehicle. A 'Q' prefix registration number will be allocated.
Scoring components
The following values will be allocated to the major components used:
chassis or body shell (body and chassis as one unit - monocoque ie direct replacement from the manufacturer) (original or new) = 5 points
suspension = 2 points
axles = 2 points
transmission = 2 points
steering assembly = 2 points
engine = 1 point
The DVLA allow for the repair of a monocoque so adding flared arches might be considered a repair to the structure but tubbing the inner arches would be probably be classified as modification as would enlarging a transmission tunnel, five linking suspension etc. The problem is that the DVLA are deliberately vague sighting that each car would be considered on its own merits rather than producing a set of rules that can be easily followed.
There is also an allowance for historical (period correct) modifications which I think allows for example for you to turn a standard MGB GT into a V8 or a standard Mini into a Cooper S replica as long as it is on the relevant owners club approved conversion list, again I'm trying to find out more about this.
Have a look at the Association of Car Enthusiasts (ACE) site:
http://www.the-ace.org.uk/ they are working hard to protect the rights we still have.
Nigel