Morning, are you sure the service interval is 5 years? As when I was looking at the Yammie 150 - 250Hp 4 stroke engines the manual stated 1000 hours or 10 years whichever one came first.
I also priced up the timing belt which was £170 + Vat and the tensioner (hope you are sitting down) was a shocking .......................... £562 + Vat from Yamaha UK
If you do decide to go ahead with the timing belt change get the bits from USA / Canada as its sooooo much cheaper and I think there is a contact on this forum somewhere. If you can't find the link let me know and I will dig it out