This is the final tally for the wheels
Mavic XC 621 £44 (These are cross-country 29er MTB rims and the max recommended pressure is 45psi or 3 BAR)
Mavic XC 621 £44
XT M756A rear hub £40
XT M756 front hub £30
Rotor 6 bolt £18 (nominal price, I actually had these spare)
Rotor 6 bolt £18
ACI Alpin DB spokes £45 (cost of 144 spokes)
Mavic UST valves £18
Schwalbe Tyres £40 (Smart Sam wired liteskin 700x35)
Preparation, build and observations
I calculated the spoke lengths by using this online calculator http://www.prowheelbuilder.com/spokelengthcalculator/
which gave me sizes of 287mm and 288mm for 32 spokes laced three cross. I opted to use 288 for all the spokes. On completion of the build this actually left a couple of threads showing (outside the nipple) on the LHS of the rear wheel and the RHS of the front wheel. So the calculation is slightly short for the longer sides and less threads than optimal are engaged in the nipples. I used the 12mm nipples that came with the spokes. Mavic however recommends 14mm spokes with these rims and this would have hidden the shortfall but of course masked the lack of thread engagement. (I believe the internal thread length is the same for both 12mm and 14mm nipples.) I didn't use threadlock or any lubricant on the threads, only some gear oil around the eyelets when truing. Because they are single eyelet I dropped the occasional nipple into the cavity of the rim when building, which was a bit of a fiddle to get out. I resorted to inserting the nipples with a spare spoke.
I disassembled the hub bearings and added some gear oil to the freehub side to lubricate the freehub body. I also packed in more grease to both sides of front and rear hubs. I then adjusted the hubs allowing for slight play until the quick-release lever was tightened. (As advised by Brucey earlier in this thread.) The reason being that Shimano do not put sufficient grease or oil in the hubs and they are usually adjusted too tight. While there was a fair bit of grease in the bearings there could have been more so this advice was good. The freehub body benefited from extra gear oil. Bearing adjustment is certainly required. I used Finish Line teflon grease and EP90 gear oil bought from a motor spares shop (1 litre costs about £6. I decanted some to a small bike lube bottle. I can share the rest with my club mates for the rest of my life). I'm a bit concerned about the lack of any real seal on the LHS of the rear hub. Previous models have had a chunky rubber outer seal which the front hubs have. I may try to get a spare to try to fit on the rear.
The rims are Mavic UST tubeless but I was using cheap Schwalbe cyclocross tyres with a wire bead. According to the retailer are tubeless ready, though this is not mentioned by Schwalbe and it is not marked on the side of the tyre. They are the new Addix compound variety and the tread and sidewall is slightly different to the older version. I used Gorilla tape on the rims and getting the tyres on was easy, and seating them was possible with an ordinary track pump. I used Stans as sealant. As mentioned above they are very difficult to get off once they are popped onto the rim.
Thanks to everyone for your help and suggestions.