Hello Paul,
20hrs from what I remember is first service when they change out the first set of oils, filters etc... and just generally check the thing is running ok. There is an intermediate after that, but I am servicing approx every 100hrs, or year and a half, whichever is sooner. My engine is currently in for its 4th service nearly 5 years on, and approx 275hours on the clock.
When I get the bill tomorrow I'll let you know what they did!! I know one thing for sure will have been the impellor since the engine hasn't been shifting enough water recently and we think it was due to one of the fins missing or something, but again, will find out tomorrow.
You about this weekend? - good swell coming in, so I'll be out on Sunday for sure
-Alex