Many thanks all for your input. I think I've heard enough horror stories to cross the E-Tec off of my list, and sufficient positives to bring Mercury/Mariner into contention.
In summary, I guess this is what you're all telling me.
Yamaha - Very reliable strong residual values but premium price. Y-COP security.
Honda - Very reliable, great warranty/back-up but again pricey.
Suzuki - Seems to be in same bracket as the above, and I already have (limited) positive experience of.
Mercury/Mariner - Certainly worth a look. May not be quite up there in terms of quality/reliability/residuals of Japanese options, but priced accordingly.
Please feel free to add further experiences, but as things stand those are the manufacturers that I'll be concentrating on at the boat show.