Put all the remaining ingredients into a large non-stick, heavy-bottomed saucepan over a medium heat and stir with a wooden spoon until all the sugar dissolves
Bring to a boil and gently simmer for 10-15 minutes, stirring continuously and scraping the base of the pan
Using a food thermometer, make sure the mixture reaches 118°C/240ºF
Remove from the heat and beat with the wooden spoon until it thickens & begins to set ( this should take about 10 minutes )
Put half of the mixture into one of the lined tins, using the back of a metal tablespoon to get it into the corners and levelled out
Add the rum and raisins to the other half of the mixture and put back over a medium heat for another 5 minutes or until it begins to thicken up again
Put the mixture into the other lined tin, again using the back of a metal tablespoon
Allow to cool completely before turning out on to a chopping board and cutting into squares
You can store the fudge for up to a week in (separate) airtight containers