Eurocode - Basic Wind Load

Clause 4.2 of EN 1991-1-4 states:

(1)P The fundamental value of the basic wind velocity, vb,0, is the characteristic 10 minutes mean wind velocity, irrespective of wind direction and time of year, at 10 m above ground level in open country terrain with low vegetation such as grass and isolated obstacles with separations of at least 20 obstacle heights.

Clause 3.4 states:

(1) The wind actions calculated using EN 1991-1-4 are characteristic values (See EN 1990, 4.1.2). They are determined from the basic values of wind velocity or the velocity pressure. In accordance with EN 1990 4.1.2 (7)P the basic values are characteristic values having annual probabilities of exceedence of 0,02, which is equivalent to a mean return period of 50 years.

This should be adequate. More information could probably be found in BRE publications (I guess).

The ones in the UK NA come from the Met Office I believe. Not sure about the non-annex ones - I had a look in my Designer's Guide to EN 1991-1-4 and couldn't find any mention of it.
I'd recommend gently advising your client that it is the law to use the Eurocodes and the appropriate National Annex (unless a given clause is not a principle) for commercial purposes, assuming the design and execution is to be carried out in a EU Nation State. I'd also probably mention that if he is concerned about the wind speed itself, I'd remind him that the many other components of a wind calculation including roughness, turbulence intensity, solidarity etc can be just as or more important. It might be worth digging around for an old copy of the building code in your appropriate country prior to the Eurocodes to demonstrate that they have/have not specifically changed which would hopefully let you get on with the design.