For a mtm ccs is the notional of the mtm leg fixed in advance or in arrears? (I refer to the fx rate used for setting the notional)Looking at standard termsheets it appears to be in advance. But i would think in arrears would be more useful! (Quanto)...

What do you mean by the 'mark-to-market leg'?The standard mechanics of a plain CCS is that it works like an IRS just with every leg being denominated in a different currency and also with the possibility that both legs earn floating or fixed interest. Thus, you just price a CCS as a short and long bond position. In that view I don't see any difficulties.

CCS quoted by brokers typically have the USD leg as MTM. eg a 5y eurusd CCS quoted at -40: This would in effect be a series of 20 quarterly cross currency basis swaplets with a -40 spread on the eur leg, and fixed EUR notional, and the USD notional re-fixing at the prevailing fx spot ratein fact, there is a quanto affect here (not perfect since not in arrears... i am quite sure the answer to my original question is they fix in advance) and so another important question is - how big can this quanto affect be - ie how big a difference in traded CCS spread is there between MTM CCS and the non-mtm CCS ?