Wife experience helm & Husband 95kg crew

yep, a small hole in the ground, suitable to piss in and sail whatever wunderboot 3000 you're recommending all by your lonesome.

Back to the OP- Alton Water, nice stretch of inland water and reasonably benign.  I'd be very tempted to get a 400 on there in your situation and use it to also explore the Orwell and Stour rivers for long fun cruises in the summer.  

 A little under the optimum weight won't be an issue in those parts, East Anglia is typically less windy than other areas of the country unless you go for the more exposed areas- even then, you'd be fine.  I've know several husband and wife combos make 400s work there.

It is a big, but forgiving boat, that you can de-power for the bigger winds, and challenge the crew with trapeze and spinnaker.  (I think Michael enjoys the challenges of symmetrical, it does take skill to fly well, but so satisfying when you get it right.)

I imagine the Alto is great for getting going quickly, but is it enough of an ongoing challenge for a crew with self tacking jib and asymmetric kite? Depends on what really attracts you.

The best idea is to see if you can have a go in whatever you decide is your short list, preferably in a well set up boat.  Don't get something just because it is a bargain.

And enjoy!

Looking at peoples suggestions I think they are missing the main point 'Your an Ideal weight for an Osprey'

I've sailed Rs's and smaller boats but find the Osprey the most forgiving in up to Force 6 compared with my 505; I own both! I'm 65kg and my crews weigh in at 14 and 19 stone, always manageable and don't remember the last time 'we' fell in!

With the new Mk V out there many Mk4's up for sale including mine. plus a few woodys. So, as you are in Kent and I'm on Sheppey maybe have a test sail or just see what the Osprey is all about.