[madolddog]

Maybe this will help - taken from the DVLA web page. Registering a rebuilt vehicle, kit car and kit conversion

If you are rebuilding a vehicle, or building a vehicle from a kit or kit conversion, it will need to meet certain criteria before the vehicle can be registered. The Driver and Vehicle Licensing Agency (DVLA) local office will assign the original or an appropriate registration mark on the evidence provided.
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Vehicles that have been rebuilt using a mix of new or used parts
In order to retain the original registration mark:
• cars and car-derived vans must use:
The original unmodified chassis or unaltered body shell (i.e. body and chassis as one unit - monocoque); or a new chassis or monocoque body shell of the same specification as the original supported by evidence from the dealer or manufacturer (e.g. receipt).
And two other major components from the original vehicle - i.e. suspension (front & back); steering assembly; axles (both); transmission or engine.
If a second-hand chassis or monocoque body shell is used, the vehicle must pass an enhanced single vehicle approval (ESVA) or single vehicle approval (SVA) test after which a "Q" prefix registration number will be allocated.
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Kit Cars
Where all the parts of a vehicle are supplied new by the manufacturer. Subject to the provision of satisfactory receipts and a certificate of newness these vehicles will be registered under a current registration mark. Kit cars which have been built using not more than one reconditioned component will also be registered under a current mark. This is subject to the provision of satisfactory evidence that the component has been reconditioned to an "as new" standard. An ESVA or SVA test will be required.
Kit Conversions
This is where a kit of new parts is added to an existing vehicle or old parts are added to a kit comprising a manufactured body, chassis or monocoque body shell. The general appearance of the vehicle will change and result in a revised description on the registration certificate. A vehicle will retain its donor registration mark if either the original unmodified chassis or unaltered monocoque body shell and two other major components are used. If a new monocoque body shell or chassis from a specialist kit manufacturer is used (or an altered chassis or body shell from an existing vehicle) together with two major components from a donor vehicle, an age related mark will be assigned. The mark will be based on the age of the donor vehicle. An ESVA or SVA test will be required to register the vehicle. Where there are insufficient parts from a donor vehicle or in cases where the original registration mark is unknown, an ESVA or SVA certificate will be required to register the vehicle and a 'Q' prefix registration number will be allocated.