It would help to establish just what you have.Sometime ago ,there was a 25 in your market made by Yamaha.Then we get into the various incarnations of the Merc 25 2 strokes.OR,are we dealing with a 4 stroke?
Since your in the uk I assume it's probably a 25ml two stroke as I don't see many 4/strokes in the merc brand as yet. If you can give me the serial number and tell me is it remote or tiller I'll post a set up guide for you. I'm on the Isle of Skye so if you get stuck I can send yu some copies in the mail