Are you using KAP files (.kap file extension)? If so, it is possible to recalibrate by hand editing the REF and PLY coordinates at the top of the file. For example, a chart with a SW corner of -35.900627,137.531018 and a NE corner of -35.538197,137.847820 (and a scale of 150000:1) would look like the following.

VER/2.0
BSB/NA=AU_SA_Western_Cove
NU=1,RA=2478,3482,DU=72
KNP/SC=150000,GD=WGS84,PR=MERCATOR
PP=-35.719412,PI=UNKNOWN,SP=UNKNOWN,SK=0.0,TA=90
UN=METERS,SD=LAT,DX=0.0,DY=0.0
CED/SE=10/6/2013,RE=1,ED=10/06/2013
REF/1,1,1,-35.538197,137.531018
REF/2,2478,1,-35.538197,137.847818
REF/3,2478,3482,-35.900627,137.847818
REF/4,1,3482,-35.900627,137.531018
PLY/1,-35.538197,137.531018
PLY/2,-35.538197,137.847818
PLY/3,-35.900627,137.847818
PLY/4,-35.900627,137.531018

PS I've written a web app for creating KAP files but it is not an editor: www.arribasail.com/tools/chartlayer.html

Gday

Thanks for the advice. When I used Notepad to edit the references I found I had 18 and not 4. I deleted all the extra ones (after 4) and rejigged the first 4 - they looked like the outer edges. Then the chart would not display. This is the file before I started editing.

! Created by GE2KAP.REX V8.0.1 - Use at your own risk!
VER/2.0
BSB/NA=Cape Otway to Gabo Island incl Tasmania
NU=AUS422,RA=7134,11014,DU=254
KNP/SC=1000000,GD=Local,PR=Mercator,PP=-41.185000
PI=0.000000,SP=Unknown 0,SK=0.0,TA=90.0
UN=Fathoms/feet,SD=ISLW
DX=100.000000,DY=100.000000
OST/1
REF/1,289,311,-37.333333,143.500000
REF/2,6848,315,-37.333333,150.100000
REF/3,6848,10698,-45.183333,150.100000
REF/4,287,10703,-45.183333,143.500000
REF/5,4773,1148,-38.000000,148.000000
REF/6,6766,1149,-38.000000,150.000000
REF/7,6766,3703,-40.000000,150.000000
REF/8,4772,3703,-40.000000,148.000000
REF/9,2780,3703,-40.000000,146.000000
REF/10,785,3703,-40.000000,144.000000
REF/11,785,6334,-42.000000,144.000000
REF/12,2779,6334,-42.000000,146.000000
REF/13,4772,6334,-42.000000,148.000000
REF/14,6765,6333,-42.000000,150.000000
REF/15,6766,9050,-44.000000,150.000000
REF/16,4772,9051,-44.000000,148.000000
REF/17,2780,9052,-44.000000,146.000000
REF/18,785,9053,-44.000000,144.000000
PLY/1,-37.3333330,143.5000000
PLY/2,-37.3333330,150.1000000
PLY/3,-45.1833330,150.1000000
PLY/4,-45.1833330,143.5000000
DTM/-471.179,0.600
CED/SE=2011,RE=01,ED=10/02/2011
CPH/0
IFM/7

I use Seaclear not OpenCPN but they are similar. I also use the navy or admiralty charts the same as Navionics use and they are not accurate inland, up estuaries etc. In this case you are better off making your own charts from the Maritime services maps. You can buy paper ones or in your case I would suggest downloading the free PDF's, converting the images to JPG or PNG and in your favourite picture programme make charts whatever size you want. Lat and long are displayed so calibrating is easy. They are extremely accurate. Maritime use them when they park alongside your yachts and check your details. Use the admiralty charts offshore where they are fairly accurate.

www.rms.nsw.gov.au/maritime/using-waterways/maps/boating-maps.html

This is my homeport.