SamuelCB:
I don't really care about the £1.50 socket but do about the £80 project so yes it's to protect my project.
If this project is in a room temperature environment then getting a fuse value above, but as close to the normal current draw as is practical, would be the be the best course of action. So a 5 A is an option, but if the 1 A difference between the fuse rating and the expected current I would use a fast-blow type.
However, the actual level of current that cause a fuse to fail can change with temperature. In general significantly higher temperatures will decrease the current required to blow a fuse and signficantly lower temperatures will increase the current the fuse can withstand. The exact details should be available in the fuse's datasheets, but it can be as much as ±25% at the extremes of the operational temperature range. So if this project is going to regularly be exposed to temperatures like -20°C or +60°C it would be a good idea to check the fuse's temperature derating curves.