you do have to inform the DVLA of a change in your engine!
http://www.direct.gov...ertificate/DG_4022054
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DVLA will need written evidence of any changes to engine number and cylinder capacity (cc). Written evidence can be:
* a receipt for the replacement engine
* written evidence from the manufacturer
* an inspection report provided for insurance purposes
* written confirmation on headed paper from a garage if the change in engine size took place before you bought the vehicle