Nemesis,
This thread got me thinking about fridge power consumption.
One of these or similar might be worth considering:
http://www.ebay.co.uk/itm/Waeco-CRP40-12...1608wt_904
Specification states its 40w and has a 23% duty cycle at ambient temp of 25C. I'm guessing that on your boat 25c would be considered tropical?
Anyway, power consumption is in the order of 220Watt-hours per day.
This would make it viable to power it from a 500w PV array ( assuming mid-winter output of 10% for 5 hours per day)
The snag is that outlay cost of around £1000 will take 10 years to recover against your existing Calor gas usage.