It should be noted that the amount of power your laptop actually draws from the charger depends on the laptop itself, not the charger. Chargers are only rated to provide a certain amount of power, so in your case, 65W (at the rated 18.5V and 3.5A). If your original charger had a higher power rating then your new one, it may heat up more then you expect (this is why chargers are only rated for a certain amount of power). Do keep this in mind, because if you are infact using one rated for a lower power, it may break faster (due to the heat).
Now, what comes out of the wall is proportional to the the voltages. Assuming a 120VRMS mains frequency, the power drawn from the wall (assuming the laptop draws the equivalent of 65W at 18.5V) would be 3.5A * (18.5V/120V) = 0.54A. The current drawn is lower as the supply voltage increases (Joule's law), so if you're in the UK, it would be even less than that. If you want to be safe and account for losses in the charger, tack on an extra 15-20%.
With respect to the fuse, with that current value we calculated, the current drawn should never exceed 3A. Why? If you were to use 3A from the wall, and assuming your mains voltage is 120VRMS, the maximum power that could be supplied (assuming no losses) would be P = IV = (3A)(120V) = 360W which is way more then your laptop could possibly use. Indeed, if it was drawing this much, then there would be a huge problem (either with the charger or your laptop).
TL,DR - use the small fuse. No laptop charger should ever draw 5A (regardless of where you live).