Since the spokes broke at the hub end the spoke length is a red herring. There are three possibilities - 1, the spokes were at the incorrect (most likely too little) tension, 2, the spokes are insufficient quality or specification for the given task (from the star pattern on the head they appear to be by Hoshi, who do not appear to make anything other than medium-strength 14/15 gauge spokes) or 3, that the wheel was subject to excessive loads for which it was designed.

Given the combination of things (i.e. 1 and 2 above) I'd suggest rebuilding the wheel with better spokes, something like Sapim Strong or DT Alpine, and using a reputable wheel builder to ensure both a high and consistent tension. I would suggest that since that's the path you've chosen you should be okay now, but it's a shame that you've had to suffer some misfortune in order to get a resolution.

I second David Ross' comments. Always best to use the best quality spokes. DT Swiss or Sapim.

The most important thing to ensure durability and reliability when building a wheel is to ensure even spoke tension all the way round the wheel on each side. The tensions on each side will be different if there is any dishing required (e.g for disk brake wheels) a wheel that does not require dishing (rear hub gear or front wheel without disk) will always be stronger than a dished wheel as the tensions can be even across all spokes. The less dishing required the stronger the wheel.

More spokes are not always better Rolf tandem wheels come with 24 spokes. Our front Rolf wheel failed because the rim broke not the spokes. As you spokes failed at the hub it could be the head of the spokes were not properly seated. We weigh quite a bit more that you and went with  32 bladed  spokes for our replacement front wheel. While we were waiting for it to be built I used a Giant PR2 wheel of conventional design with 28 spokes.  If the original wheel was built with too short spokes I wonder what else they messed up? PS we did chase the Sun a 205 mile ride on a 24 spoke Rolf rear wheel. See photo!

"On top of all of all the other valid comments, and in my opinion, the wheel set you have is not strong enough for the area you live in. think about acquiring a set with more spokes or much stronger rims. I had the same problem with Dawes Double Edge. straight out of the shop i broke three spokes in the rear wheel in a hilly twenty miles. I binned all the spokes in the rear wheel and rebuilt it with DT Swiss. ( it's 36 spokes but adequate for a 26" wheel and its still ok after 30 odd years,)" 

I would like to think wheel rims and spokes have improved over the last 30 years.                  

Some good points made in  the comments above.

We're a similar weight team to you and I quite happily use 36 spoke wheels - modern rims and spokes are very strong.  In my case, Ryde Sputnik rims with Sapim Strong spokes on the rear and Sapim Race on the front.

I think your Mavic rims come into the same 'bombproof' category as my Ryde Sputniks.  Rebuilding with good quality DT Swiss spokes will make the wheels more than strong enough.  I fitted a rear hub motor to my Orbit Velocity. It was supplied with a 2 cross lacing pattern and plain 12 gauge 'Chinesium' spokes.  Several spokes failed at the elbow within about 500 miles.  The bike itself is probably over 35Kgs now with rack / panniers / tools / motor / battery etc. I rebuilt the rear wheel with the Sputnik rim and Sapim spokes laced as a single cross (the motor hub flanges are too large for any other lacing pattern).  That wheel has been fine for a few thousand miles since.  I laced the front wheel as a normal three cross pattern and it is fine too.

IMHO those Mavic rims with DT Swiss spokes will be perfectly OK.

Thanks everyone for all the really helpful comments, I will try and address all the good advice.

Reassuring as seems that I have taken the right path in having the wheels rebuilt with DT spokes. With regards to the short spokes I think that the real concern is the competency of the wheel builder, this is why I would not let the supplier fix the wheels for me, they did agree to pay for the rebuilds which I thought was fair.  I was concerned about the front wheel failing dramatically and will feed back to them on the poor quality spokes.

Would like some nice factory built wheels like the Rolf's, but will get the most out of this current wheel set for now. Obviously a lot less choice in tandem wheels due to specialist area. Have had thoughts about a deeper section / aero type rim but knowing what could be used safely is an important consideration.  The trend now is for wider rims and as far as I'm aware I can't find a suitable alloy rim  and going carbon I would really just not be sure about for a tandem.

I'm going to see how we get on with the rebuilt 36 spoke wheels, as per advice I think that this will be sufficient, if the failures continue will have to look at stronger wheels but hoping to avoid this.  My other tandem (for sale if you are interested!) has 48 spoke wheels which are a little over-built for our purposes but obviously very reliable :-) 

I'm type talking, forgive any typos