Duncan
Tried your idea but unfortunately it didn't work
However it did set me thinking and I've found a technique that works using your idea as a basis.
1) Delete all but the last image from the card and rename the last image to the file number one before you want to start with - in my case 2083, and rename the folder that it is in appropriately - 120CANON in my case. (in continuous numbering Canon store 100 images per folder starting at 100CANON for images 0001 to 0100 then 101CANON for 0101-0200 etc so images between 2001 and 2100 would be in folder 120CANON)
2) copy this folder to you PC
3) then format the card in a card reader
4)
BEFORE putting the card in the camera, set the file numbering system to "auto reset" and then insert the card.
5) take a picture and it will have file number 0001 in folder 100
6) put the card back in the card reader and copy across the folder & image created in step 2
7)
BEFORE putting the card back in the camera set the file numbering system to "continuous" the insert the card
8) take image number 2084
Thanks for starting me thinking Duncan