leave it as a 1.2 on the log book mate, cheaper tax, but you've said its insured as a 2.0 conversion............result!
my mi16 205 was insured with every mod declared, and still a 1.6 on the log book, nice cheap tax
You still keep the same tax rate even if you do change the log book to a 2.0 so there would be no difference in tax.