4 digit engine numbers

[Ian F]

From the Gov website:

When you need to update your V5C
You must update the details on your registration certificate (V5C) to tell DVLA about:

mistakes on your V5C
most changes you make to your vehicle
You cannot change tax class by updating your V5C. You do this by changing your vehicle tax – even if you’re changing to a tax class that’s exempt from vehicle tax, for example ‘disabled’.

This guide is also available in Welsh (Cymraeg).

Changes you must put in your V5C
You must update your V5C when you change any of the following:

colour
engine
cylinder capacity (cc)
fuel type – for example, if you convert your vehicle to run on electricity
seating capacity
weight of a large vehicle, for example goods vehicle or campervan
chassis, monocoque bodyshell or motorcycle frame, if their size or appearance is different from when your vehicle was made – find out about structural modifications

[Ian F]

The two extracts below are from the FBHVC, regarding the new vehicle registration regulations.
Essentially it now seems that replacing an engine, since it is a change to the V5C, now puts the vehicle into the "repaired and restored vehicle" category, and must be notified to DVLA


Repaired and restored vehicles
There will be no requirement to notify DVLA where a vehicle has been repaired or restored back to its original working standard, providing:

worn-out components or body panels are replaced on a like for like basis
the appearance of the vehicle is the same as when it was originally manufactured
Vehicle log book (V5C) details are not affected.

Telling DVLA of changes made to a vehicle: There is a legal requirement to notify DVLA of any changes made to a vehicle which affects the information shown on the V5C registration certificate (log book) such as replacing the engine. Policy document INF318 sets out the detail of this.