D & C making it up as they go along again!
They can only charge for an increase in the number of weapons held. A one for one is not chargeable according to the HO guidance.
Chapter and verse...
Home Office publication "Firearms Law: Guidance to the Police 2002"
10.49 Form 101 should be used for
applications to vary a certificate. The
certificate holder should submit their firearm
certificate, along with the completed form
101, and, where appropriate, the fee, to the
police force headquarters. A fee is payable
only in respect of variations which increase
the number of firearms to which the
certificate relates: if the holder wishes to
dispose of one firearm and replace it with
another at the same time then no fee is
payable (see paragraph 10.51 below for
further information).
10.51 A variation is always necessary if a
certificate holder wishes to change one of the
firearms, even if they wish to purchase one
identical to the one they are selling (Wilson v
Coombe, Queen’s Bench Divisional Court, July
1988). “One for one” variation refers to
firearms that are authorised to be acquired
at the same time as one is being disposed
of. For practical purposes this is normally
taken to be within seven days. The use
of authorities which can be kept open
indefinitely should be discouraged.
Applications for “one for one” variations
should be made by the certificate holder
submitting their firearm certificate together
with a completed form 101 to the police
firearms licensing department. Such variations
are processed free of charge. In most cases,
it will not be necessary to re-examine the
applicant’s circumstances. Further inquiries
will be necessary, however, if for example
the application is for a change of use or
for a full-bore firearm when the holder’s
shooting club only has facilities for
small-bore shooting.
The first section in bold suggests that it is not only slots that are "filled" that can be the subject of free one for one variations, but also unfilled slots. As long as you can prove good reason to hold, they should do it FOC.