The official B & G bearings for the wind angle and wind speed are quite expensive. Why not buy some 3rd party bearings for 10% of the original price.
These should match the 213 and 496 Masthead units for Brookes and Gatehouse.
B&G part numbers 213-10-004 and 213-10-038 (Contact me for more information, or just google the part number for a local shop).
Replacement is quite easy, remove the MHU from the mast, and there is one screw (secured behind a sticker) to remove to seperate the top and bottom parts. just replace the bearings and your windspeed sensor should work fine for a few more years.
For the 213 MHU, take a look at the picture above, the screws to open the unit are marked by the red arrows.
This is a pdf with all the different parts and their numbers.
For the 496 unit, check out the pictures below (Thanks to JanW).