i'm not using the CSS rims, just the normal ones.
Okay! That helps a lot; thanks.
the pads i started out with were shimano pads that are fitted as standard on the Sherpa. i changed to a cheap set bought from Asda
I would definitely suggest a change to Kool-Stop Salmon pads as the next step to try. If you're using (or still have) the Shimano pad holders, Kool-Stop make a Salmon cartridge pad insert that slips right into the same holder, replacing the original Shimano pad. Much nicer to carry only the cartridge inserts on-tour, as they require less space in your kit. Given their low rate of wear and how "kind" they are to rims, they will last you a long time, so are a good value for the money.
with regard to the lighter Mavic rim , which is about 100grams lighter than the Rigida Grizzly's , would that make a difference in this case ? in other words, would they be too light a rim for the spokes that came as standard with the bike ?
Yes, I would probably go to a 15ga (1.8mm) spoke with the Mavic instead of the 14ga (2.0mm) spokes used on the Grizzly.
But first, I would have a good talk about your intended use, your own weight, the weight you intend to carry, and the roads you'd be using the bike on. The Mavics would simply build-into a lighter wheel overall, and that is a huge consideration. Those are all things I would want to know before building a set of wheels for someone -- and the same questions I ask myself when building my own.
But wait...there's more!
It doesn't appear possible to do a simple "rim swap", as the ERD (Effective Rim Diameter) of the two rims differs. ERD is "...the rim diameter measured at the nipple seats in the spoke holes, plus the thickness of the two nipple heads. The E.R.D. is needed for calculating the correct spoke length" (ref. Sheldon Brown, http://sheldonbrown.com/gloss_e-f.html
According to SJS Cycles ( http://www.sjscycles.co.uk/mavic-xm317-26-%28559%29-mtb-all-mountain-rim-silver-prod10607/
) the Mavic XM317 has an ERD of 539.5mm (for the rim-brake, non-disc version; SJSC carry only the 32-hole version at present).
SJS Cycles say the Rigida Grizzly has an ERD of 534mm ( http://www.sjscycles.co.uk/rigida-grizzly-26-%28559%29-mtb-rim-black-36-hole-prod18844/
[Please note: The ERDs listed for these two rims vary widely depending on vendor and spoke-calculator listings; be sure to check the ERDs are all for rim-brake versions.]
According to SJSC's listings, the spokes used to build the Rigida wheel would be too short for re-use in the Mavic. The thought of a direct rim-swap may not have crossed your mind, but I thought I'd mention it, since you had rebuilt the Felt wheels reusing the same rims.