I don't know where to find one of these connectors. Generally, parts like that are specific to the car manufacturer and you can't buy them elsewhere, which is a real nuisance. If Rover still existed they would have expected you to buy a new loom. The practical solution is to cut it off and fit 5 seperate bullet connectors, or get a different type of 5 pin plug and socket from an auto electrical supplier. See here for example:
http://www.polevolt....Connectors.html
The 6 way connector looks useful for this.
Having said that, it is not impossible that someone here will have a connector, or know where to get one.
As to your fuse rating, having LEDs does not change the rating needed to protect the wiring. I suspect that the connector was not making good contact, and the high resistance did not allow sufficient current to pass to blow the fuse, while allowing a local hot spot to develop.