I would suggest that your "solicitor" analogy is somewhat different to written advice from the Home Office who are after all the government department responsible for firearms legistlaton, the Police and the CPS.
It would be a foolhardy DPP to instruct the Crown to prosecute against written advice issued by the Crown.
As requested, here is the letter:
Reference: T18837/12 20 December 2012
Dear
Thank you for your e-mail of 12 December in which you sought clarification as to whether a sound moderator unattached to a section 1 firearm is controlled under the Firearms Act 1968 (as amended).
Although not specifically defined by name in the Act it is the view of the Home Office that sound moderators and suppressors intended for use as parts for either a section 1 or section 5 firearm are subject to section 1 licensing controls and a firearm certificate would be required to manufacture, possess, purchase or acquire them.
Further advice relating to section 1 firearms licensing control and firearms certificates may be obtained from your local police Firearms Licensing Department.
I hope that this deals satisfactorily with your enquiry.
Yours sincerely
I Newman