Noddy, would be worth checking with either your local Merc dealer or Julie at Steel Developments, who also sell new props. She is very helpful, just give her a ring. Do mention Ribnet, as they some time post on here.
The last prop I bought from a Merc dealer cost me £115 and that was 1995, thankfully its not a Yamaha outboard
Take your existinp prop off and see what size that one is, but I think 12" pitch may be to small unless you are planning to keep it just as a spare and the price is really cheap
http://www.steeldevelopments.com/Mer...ropellers.html
Pete