Excellent, Dave, excellent.
You have me confused now. A mortice (noun) is spelled with a "c", in common with licence, practice and notice.
To mortice (verb) is spelled with an "s" , as in to practise, to license. To notice is an exception, it seems to me.
So when we come to a mortiser, where are we? I would spell it with an "s" as it as a machine that does something. A verber? Ouch! But a mortiser is, itself, a noun and I am hoist by my own petard.
I think I'll continue to call a mortice a mortice and a mortiser a mortiser and just have another glass of wine.
S