I replaced the actual shower unit with a new one, pratically identical one
from
Wickes/Triton.

How sure are you that the "identical" replacement had the same power rating
as the previous one? I strongly suspect your replacement is more powerful,
as a fused one is likely to be at 30A, giving a maximum of 7.2kW. Of course,
it is possible to get bigger fuses, but they aren't that common. Don't go
fitting a bigger fuse unless the cable is found to be OK for the larger
current.

Should I be concerned about this?. I can easily replace the fuse and
hopefully catridge, but it has scortched some of the bit the fuse plugs
into - can this be replaced.

Yes, but fuse boxes always seem to do this. After a few blows, they look
like melted Mozzarella on a pizza. Consider replacing your consumer unit
with a modern one, with RCD and MCB protection.

Christian.