immersion...now its a combi boiler....do you think it may be on the 15 one now
Possible, if the new boiler was fitted in a convenient location to use the old immersion heater feed. Just pull that fuse and see if the boiler stops working......
in fact what would be the problem fitting one of these instead of a cartridge fuse of 5A rating
You mean a fuse intended to fit a 13A plug in your consumer unit?
The fuses for plugs are to BS1362, while those for the consumer unit are BS1361. They're different physical sizes.
The BS1362 fuses for plugs are all the same size from 1A right up to 13A since they all have to fit the same fuse clips in a BS1363 plug. The BS1361 fuses are different sizes depending upon value. While the 5A isn't far off the size of a BS1362 fuse, it's still different.