I agree that this looks like an Exeter mark but Miles Harrison's excellent book "Exeter & West Country Silver" does not illustrate a PF mark. One possibility with the right initials is Peter Flood of Exeter who sent items for assay between about 1794 and 1801. To back this up the lion passant seems to match the one used from 1798 to 1804 (see the
Exeter Lions page).
Phil